300tdi jap import sub woofer not working / connected

So this is only my second post after my introduction, so I expect to get some stick
I have been reading this forum for a few months now and posts have helped me to fix many jobs with great success, so thanks to those that wrote about "door lock springs, rear electric windows dry solder on ECU, inoperable rev counter coz cable came off alternator".
I now need to ask a question because I have been reading for days, but can't find the exact answer:
I have a 98 300tdi County Jap import where the rear door speakers don't work. When I bought the car it had an aftermarket head unit fitted, but the sub woofers in the back door have not been connected to the head unit RCA pre out.
The head unit is connected using the vehicle grey ISO for power ect, and 8 speaker cables Into a second ISO connector for the front and rear speakers. There is a third Brown vehicle ISO connector that is not being used that I read is for dash switch gear.
I have read that some people have used a fourth black ISO connector that has a black and red cable to connect to the sub out, but my vehicle does not have this fourth ISO.
I have also read (Discool) say that I would need to by pass my vehicle amplifier that lives in the passenger footwell, and this is where I get lost. I have found the amplifier which has already had the white 18 cable socket removed (see photo). The previous owner has cut off 8 speaker Cables from this white amp socket and has spliced new cables to them to feed the speaker ISO at the head unit.
So I am left with 10 single coloured (not striped) cables by the amp. My question is which ones do I connect to the single RCA from the head unit? I am not even sure if this is correct?
Does anyone know the solution please? Many thanks

KNine said:

So this is only my second post after my introduction, so I expect to get some stick

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Feck off, noob!!

Only kidding. On my ex-Jap, I ripped the whole lot out as it was a wiring disaster!

So can we take it the four existing loudspeakers are functioning ok?
If that is all the
spare wires u have then u must find out if any go back to the back door bass loudspeaker. You will need a buzzer and battery or a multimeter to do that which I'm sure you know, u will need to find a pair for the audio plus two wires for the two 12v supplies for the amp so 4 wires in total.
Maybe easer to test backwards as the 4 wires (one pair two singles) are already at the amplifier.

auf_wiedersehen_pet said:

Only kidding. On my ex-Jap, I ripped the whole lot out as it was a wiring disaster!

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Thanks for the reply. I dont need to spend too much on the sounds in this car, just want to get some Bass, no need to change too much. By the way, your build was one of the first threads I read on this site, read the whole thread, very inspiring.

discool said:

So can we take it the four existing loudspeakers are functioning ok?
If that is all the
spare wires u have then u must find out if any go back to the back door bass loudspeaker. You will need a buzzer and battery or a multimeter to do that which I'm sure you know, u will need to find a pair for the audio plus two wires for the two 12v supplies for the amp so 4 wires in total.
Maybe easer to test backwards as the 4 wires (one pair two singles) are already at the amplifier.

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Glad that you have answered Discool, I get the impression you are the stereo / electrics man on here. yes the four existing speakers are functioning fine. I have not managed to find any other spare wires.
I will have a go at buzzing back on the weekend as you suggest, I assume it is ok to cut the remaining 10 wires off the white block as the Amp is no longer utilised.
Once I find 4 wires, how do they connect to the RCA cable?
Thanks for your help

they wont, youll need an amp for the sub, the speaker wires (once you identify them) will connect to the amp output, the amp will connect to the head unit via an RCA cable.
RCA is a line level signal, it will not drive a speaker directly.

may I suggest popping the sub out have having a gander at the wire colours?

also, I would go chopping it off without insulating the cables, chances are at least one of those is live.

KNine said:

Glad that you have answered Discool, I get the impression you are the stereo / electrics man on here. yes the four existing speakers are functioning fine. I have not managed to find any other spare wires.
I will have a go at buzzing back on the weekend as you suggest, I assume it is ok to cut the remaining 10 wires off the white block as the Amp is no longer utilised.
Once I find 4 wires, how do they connect to the RCA cable?
Thanks for your help

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The colours look familiar in the connector I have a list, somewhere on what circuit they are which i'll try and find. but I'd leave them as is just use what u need, there is some room below the head unit to stuff things away. I manage to stuff a birds nest of wires an connecting blocks away.

I had to tap into the s audio feeds to feed a sub amp I fitted under the front passenger seat to power a 10" loudspeaker in a box sitting on the boot floor so the easy way was to pick-up at amp on the back door and so I had to 'suss out' what was what and below is what I found at the terminal block on my disco.

There are four circuits to test at the rear sub.
1. 12+v amp power taken from the head unit supply, (light green/orange wire at the amp
2. 12+v amp switching wire, switched by the head unit (purple/yellow at the amp
3. Audio signal cable (black & white wires in a screened cable)
4. The local earth wire is black at the amp).
As stated before 1. Is when the ignition is switched on and 2. Is when the head unit is turned on.
So get the supplies to the amp right first and if still not working then look at the amp and the loudspeakers for a problem

The audio cable had two wires red & black and it's own connect, the two 12v supplies u may find in the multi block that u have.

The audio will be connected to the head units pre-amplified output via patch lead and the main 12v to the head units permanent supply and the switch to the head unit switch supply output usually a blue wire.

If you just can't find the wire at the head unit end then u will have to install them from the unit to the bass amp in the backdoor, easy just a half hour job

discool said:

The audio cable had two wires red & black and it's own connect, the two 12v supplies u may find in the multi block that u have.
The audio will be connected to the head units pre-amplified output via patch lead and the main 12v to the head units permanent supply and the switch to the head unit switch supply output usually a blue wire.
If you just can't find the wire at the head unit end then u will have to install them from the unit to the bass amp in the backdoor, easy just a half hour job

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I don’t know why I can’t find the red & black in its own connect, but I probably will install from the bass amp in the backdoor as you suggest. I wish it was a half hour job, these things seen to take me hours. Thanks for the info.

timtheenchanteruk said:

they wont, youll need an amp for the sub, the speaker wires (once you identify them) will connect to the amp output, the amp will connect to the head unit via an RCA cable.
RCA is a line level signal, it will not drive a speaker directly.

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I believe the sub has its own amp at the rear door

KNine said:

I believe the sub has its own amp at the rear door

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It has, remove the grill and all the other screws u see and the speaker/amp unit will pull out, the pair of loudspeaker are bolted to a plate along with the amp is a single compact unit, no additional wire required .

You cant find the audio wires 1. the have been cut back 2. they never existed as said not a great problem if they are not there.

So, managed to get the sub working yesterday pretty much as you suggested Discool, so thanks for all your help.
Here are a few notes for the record:
1st picture is the sub connector at the rear: wires are: orange/green, purple/yellow, black earth, black in a sheath, red, black.
2nd picture is the from amp 18 cable connector. As you can see I cut the red, black, and purple/yellow off the connector. I then connected the red and black to a single RCA, red to the core and black to the sheath (photo is temporary block connector), which in turn plugs into 1 x white head unit RCA for sub. I then connected the purple/yellow to a length of cable (brown) that connects to yellow on my head unit (switched power).
Everything works as it should with regards to switching on radio with ignition and sub working with no interference so very happy.
3rd picture is cables at head unit.
I did initially have green/orange to yellow at radio, and purple/ yellow to red, but the radio lights would come on without ignition. I then disconnected the orange/green, and that resolved the power being on without ignition, but when I did turn ignition on then off, the radio stayed on. So that is when I swapped the purple/yellow to feed the yellow at the radio as stated above, and without using green/orange at all.
Thanks once again.

I couldn't work out how to post multiple pics, so here are the other 2

KNine said:

I couldn't work out how to post multiple pics, so here are the other 2

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Here is third photo:dizzy:

Well done, as u have found it's all logical stuff.

U don't say what head unit u have but did u have to adjust the head units RCA output gain (Hz), the output should be mono to get a decent bass and insure the phase is normal (push) not reversed which will sound a bit flat, but if it sounds OK to u then no need to bother.

discool said:

Well done, as u have found it's all logical stuff.

U don't say what head unit u have but did u have to adjust the head units RCA output gain (Hz), the output should be mono to get a decent bass and insure the phase is normal (push) not reversed which will sound a bit flat, but if it sounds OK to u then no need to bother.

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It's a Sony CDX-DAB500U, not expensive but does everything I need, Dab, IPhone USB, CD. I have set phase to normal, but didn't know about the mono bit so will have a look. it does sound OK though. Cheers.

Just had a look yes it has all the facilities u will require. U can play around with the low pass filter to see what suits u mine is set at 80Hz, I only play CDs, but u may find when using the radio that the frequencies don't go that low so no sub bass, so will have to raise it to 100 Hz or above.

It looks like the your setting for mono is automatic, my Pioneer unit had to have both L & R sub channels paralleled when connected to my 200 watt per channel Kenwood stereo amp which has a single mono input, when it's bridge for sub woofer use.

So, I now have a 400 watt sub plus the 200 watts from the head unit if I ever needed 'loud' which I never do.