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.

 

I have a wiring diagram thank you. it doesn't work very well though when the outfit one is working on has two or three different sets of wiring from different years because the color codes do not match.

I have spent all summer in my spare time checking, evaluating, and cleaning every ground wire I can find on this puppy. I then cleaned every connector that I could get my hands on. Last night I reassembled it and hooked the battery up. Lights are still ON! This morning I traced the wires from the Backup light switch and they run to an Ignitor assembly on the drivers side fender. This gets power from the Alternator. Two of the wires have been damaged before and one has been pinched. In researching the part number of the Igniter Assembly (89620-35310) everything indicates that it would be for a 1992 or newer. The damaged wires aside, would having an igniter assembly from the wrong year and possibly too hot or cold cause the backup light problems and or my melting of the dash cluster?? I am at my wits end here.

 

I spoke with someone with more knowledge than I have and they suggested running new wires from the lights themselves to the actual source for power. It got me to thinking. I decided to retrace the wires going forward from the taillights to the cab. Everything looked really good up to the entrance to the cab. The boot going into the cab was in good shape so I moved forward. Feeling through the carpet I could not decide if the wires went forward or across the cab. So, I pulled the seat out and lifted the carpet. There was a connector there. It was badly corroded so I pulled it apart and cleaned it up. From there the wires went forward through the threshold cavity and then into the cavity behind the ECM. My intent of this search was to find the starting point of the wires to the backup lights so that I could do as you said. So I kept on going. Through the fire wall and into the engine compartment. the wires were in a wire loom and disappeared under the intake manifold. On a whim, I pulled some of the peripheral parts out of the way and uncovered the wire loom, pulled the wires out and started looking for the one wire that I had been following. In doing so, I found that wire melted to 4 other wires. Severe enough that all of the insulation was melted through and the copper wires were stuck together. I fixed all of the wires and then checked to see if the lights were off. Hallelujah, they were off!

Two things to note for anyone who may have similar problems down the road.
Wiring diagrams are great for telling you wire colors and what they feed. But, they do not indicate splice connectors. The connector under the seat is not on any diagram. Had I not decided to investigate the wire routing I would not have known it was even there. I don't know if it had anything to do with wires melting further down, but I am sure that it did not help with continuity. I have encountered several such connectors in my search for this problem.

I did not think that I would find melted wires inside of a wire loom in an area that does not get overly hot. The loom itself did not show any damage at all, so I did not originally feel the need to investigate it. I learned a big lesson from this one. Looks can be deceiving.
I'll be putting everything back together in the morning. I hope to be driving it again in the afternoon.