If it only blows when the trans is in reverse it has to be a short somewhere between the switch and the backup lights. From the wiring diagram I don't see anything on that circuit after the switch except the lights. It could be the switch grounding out internally but I'd suspect wiring.
There should be plugs on the two wires going to the switch about 2 inches from the switch on the trans. There are two switches, the reverse switch has red wires going to it. The wiring diagram isn't clear enough for me to tell, but it looks like the red/blue stripe wire is to the lights, and the red/black stripe is from the fuse box. If you disconnect just the red/blue stripe wire and it still blows the fuse then it would be a bad switch, if not then there is short somewhere in the harness connecting to the bed back towards the lights. Another confirmation of it being a bad switch is if the fuse doesn't blow and the lights come on when you connect both wires together. I'm guessing you will have to trace the wire coming from the backup lights through the bed harness and back to the switch. Somewhere in there will be your short if its not the switch.
My 79 had wiring issues all over it when I bought it due to a cooked main alternator wire in the big harness bundle going across the firewall. It shorted to the wire between the reverse switch and the fuse box and the truck wouldn't shut off with the key any more. The alt wire was feeding power right from the bat into the reverse light circuit and also back into the fuse box. That switch is on the same circuit of the fuse box as the coil I believe, fuse blows=no spark=truck dies. It took a couple hours of unwrapping the harness and looking things over to find it but it wasn't too complicated.
_________________
95 Isuzu Trooper Daily Driver
86 Isuzu Trooper reliable backup
77 LUV 2wd stock beltway blaster (resting)
79 4x4 LUV project: 2.6L, 5spd, 31s (eventually)
MEPR: Man, my 4x4 makes all other LUVs look good
Login
FAQ
Search
