Audio help

Help needed.... when I purchased my car it came with and alpine headunit all ready installed but not all the speakers are working.
All the fronts work fine but there is no sound from the rear doors, when I fade the sound to the rears from the headunit the sub in the boot comes on. As far as I can see it has been wired up correctly with the proper connectors, I have checked the speakers and they are connected and not rusty.
Please help.

Are you sure its wired correctly? The sub should come up when you turn the sub up not when you fade to the back.

Stv123 said:

Are you sure its wired correctly? The sub should come up when you turn the sub up not when you fade to the back.

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The connections behind the head unit are shop brought iso connections and no wires have been cut, when I say sub in the boot I mean the factory fitted sub.

I think there is a setting for the sub in the head unit. but cant see why the fade should alter that.

Stv123 said:

I think there is a setting for the sub in the head unit. but cant see why the fade should alter that.

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When I brought the car it had the standard harman kardon speakers but an after market alpine head unit, I could hear the sub in the boot floor crackling so adjusted the fade and that's when I discovered when faded to the front the front doors where on but faded to the vmback and the sub was on and not the rear doors.
I'm not if it has anything to do with the amp under the passengers seat?

If the HK speakers are the originals, there is a very high chance they have had it - so you may want to test them to eliminate them from your fault finding before going through the wiring if you haven't found anything obvious yet.

As above. I also suspect the rear door speakers are dead, which is pretty common, the sub will probably need a new driver too, as those are also known to disintegrate with age.
The sub driver is cheap to replace. The rear doors can take standard 6 ½ inch speakers, if new screw holes are drilled.

I read somewhere the amp is in the boot fed by a fiberoptic cable but can’t say I have ever worked on it.

What year is it?
here is the diagram for mine, good luck.