I have an 84 Toyota pickup, SR5 4x4 with 5 spd manual trans. The former owner removed the 22R motor and put in a 22RE. Everything else is still stock.
This winter I noticed that the backup lights came on and stayed on all of the time when the key is turned on. Then it happened. One day I smelled smoke under the Dash. I pulled the Dash apart trying to find the problem but there were no melted wires that I could see. I got to looking at the instrument cluster and the ground was burned between "A-12" and"B10". It was completely fried. The truck still ran, but none of the gauges were working. The backup light switch seems to be working properly, but even with the switch disconnected the lights remain on when the key is turned on. I was able to locate a new Instrument cluster from a wrecking yard and installed it. Then started the truck and immediately smelled smoke so I shut it off right away. I'm attaching a picture of the original cluster and the new one after I smelled the smoke. It did not burn anything to speak of, but it did get hot enough that the circuit started to separate from the board.

I have tried to trace the wires from the backup lights through the dash and cannot find any evidence of wires getting hot or melting throughout the dash. Before the cluster melted, last fall the fuse to the gauges blew. I replaced it and everything seemed to be fine. this time around the fuse did not blow. It just melted the cluster. On the Circuit board, the circuit between A12 and B10 is continuous, which would tell me that the grounds connect to the same point somewhere. But I have not been able to find out where that is. I have traced the wires into the rear light housings and I cannot see any evidence that they cracked or shorted out. I did install a new backup light switch just to be sure, but the lights stay on even with the switch disconnected.
I'm not sure if I have two separate issues going on here. One with the cluster and one with the backup lights themselves.

Any ideas as to where to look would be very helpful. Thanks

 

My advice is buy the Toyota Electrical Diagram book from Toyota that covers your year ,make & model. I did so & it saves me a lot of time , has not much is stock on my truck any more& I make lots of mods happen!

 

You’ve got a major short somewhere. A wiring diagram is a must have item.
Chasing gremlins in 40 year old wiring can be a major headache.

Check all harness areas that go through the firewall. Check all plastic connectors for melting.

I had a problem with my directional switch flashing quickly. Found a melted plastic connector attached to the lower steering column. Not sure what caused the problem in the first place, but replacing that connector solved my problem. This is why you need a wiring diagram.