Sorry, I didn't read your other post about the wire colors. Here's what I'd do.

1. Set the DVOM to OHMS and connect one lead to the circuit side of the fuse and the other side to a good ground. The best ground is the negative battery post but most meter leads aren't that long so try to find a bare metal spot in the door frame etc.

2. Disconnect the brown wire going to the dimmer swtich and test for resistance (continuity).

At this point you have isolated one part of the circuit and removed the ground so there shouldn't be any way to complete the circuit (resistance) unless the circuit is shorting to ground. Keep in mind that a short can be to metal or to another wire that makes a ground connection. If no resistance is detected then the circuit is good and you have to keep searching. Also when testing the circuit, jiggle the wire just in case there's intermittent contact. Remember, you are not checking resistance of the wire. Most wires will have some resistance. You are looking for that sneaky short that's not supposed to be there. That's why your leads are at the fuse and the battery post or a bare spot on the frame, NOT at the fuse and the end of the wire.

If you see something like 0.1 Ohms of resistance after you have removed the ground then that is bad. Ohms law states that
Voltage (volts) = Current (amps) X Resistance (ohms)

So Current = Voltage / Resistance
and with voltage = 12.8 and ohms =0.1 then
current is 12.8 / 0.1 = 128 Amps and your 5 amp fuse blows which is what it's supposed to do.

So remove the ground, connect leads at the circuit side of the fuse and the negative battery post and jiggle the wire. No resistance (OL or -1 on the meter) then the circuit up to that point is good. Resistance detected then there is a short. Yah, trying to find a short is a real pain.

 

I just remembered another technique that doesn't require a DVOM. If you have a bucket of 5 amp fuses or better yet, if you can wire in a 5 amp resettable circuit breaker then by using your wiring diagram you can start by disconnecting various parts of the circuit, applying power, and checking for the blown fuse. When the fuse doesn't blow then you've isolated that part of the circuit that's causing the problem. Then you have to check whether it's the wire that's shorting or the load or if an aftermarket addition is drawing too much current for the 5 amp fuse to handle (ohms law, less resistance = more current).

The service manual's diagnostic section should also give you some help in isolating the problem.

 

Thanks for the info. Dad went back to look at it last night after I'd retired for the day, and I think he's gonna setup the dimmer controller on the bench and test it a few ways to see if there's something I missed. (Which is where the brown wire goes into).

I'll see here in the next few days if he turns up anything.

Thanks again!