Although I'm not brilliant with a digital multimeter....that's what you need to get yourself.
Set it to ohms and start probing.
Disconnect the indicator wires going to the rear at the engine bay if the fuse still blows it's reasonable to assume the wiring is ok to the rear, then disconnect the front indicator circuit and so on and so forth.
Slowly you will narrow it down to a certain wire.
Trouble spots tend to be where the loom exits the rear chassis and ends up behind the internal lamp cover. The wiring loom then runs along the top of the rear cross member to the ns lamps. Wiring damage is possible here too.
A dmm will show which wire is going to ground and it may take you ages to find it and it may not but without one it will be frustrating.
As I understand it ...
A circuit such as a brake light for example will have no voltage going down it untill the brake pedal and thus the switch is activated. This voltage needs to get to the bulb right up to the actual filiment unhindered to operate as it should. 2 things can happen when you have a wiring fault on the 12v + side of the bulb.
Note + side not - (earth) side.
A dull bulb (note- we are only concerned about wiring issues with my explanation as you have renewed all the components in your circuits) caused by a bad connection/ corroded wires.
This is what a dmm will find when it's set to ohms also known as resistance to learners like me.
If you had a length of insulated wire that was in good looking condition and put the two probes off the dmm at either end you would get a low reading (0.01 or something?) Telling you the wire is good and has a low resistance to voltage running through it.
If you got a high reading instead then the wire has a high resistance to voltage flowing through it, peel the plastic insulation off the wire and you would find broken strands or corrosion not visible to your eye without the dmm.
No reading at all says the wire has completely broken inside.
If you get probes with wire spikers (sharp pins basically) and leave one probe still connected to the wire and spike probe with the other you can find the exact position of the break.
Second problem , which I guess you have is shorting to earth.
What happens in that situation is that unlike the explanation above where the bulb is just dim from high resistance the voltage is escaping (somewhere) before it reaches the bulb and (usually) touching metal somewhere (which is earth) or touching another earth wire.
To find this you use ohms again on your dmm.
Using the cable example again but this time the cable is lying it's full length on a sheet of steel (or a metal part of your Landy if you like) when 12 volts positive is run through a fuse down the cable from a battery and the negative from this battery attached to the metal sheet , as long as the other end of that cable doesn't touch the metal sheet the fuse will not blow if the cable insulation is good.
However, if the insulation isn't good because it's damaged in some way and the inner wire strands touch that metal sheet the fuse will blow.
This is your problem I suspect.
Now, because you don't have a nice wire lying on a metal sheet that's easy to check visually for insulation damage then you need a dmm.
An example check on your truck would be to disconnect the wire going to whatever rear indicator that blows the fuse, completely from all the other wires just before it enters the chassis at the front and completely from all wires at the rear before it splits off to the ns indicator. You now have just one single wire with nothing else connected to it and two ends.
With the battery connected and with your dmm set to 'bell' if it has it (audible beep when a circuit is complete) touch one probe to the wire end and one to a good earth.
If the insulation is good you will hear no beeb. If it Beebs you have found the problem, voltage which needs to stay inside the insulation, is not and is touching metal or another earth wire (same thing) somewhere along it's length and blows the fuse.
Your fortunate in a way because the fault is permanent and not intermittent. Imagine if that wire running through the chassis had the damaged insulation but only moved and touched metal if you went over a bump say but moved away from the metal when there were no bumps so you would be trying to find a fault that wasn't there when you were parked up.
