Alright first off I'm posting from the mobile app so I apologize if punctuation Isn't that great.

The car is a 1986 Trans Am started as a lg4/700r car. It now has a 5.3/t56 4th gen fuel tank Pocket harness ect. I didn't notice the issues until after the car had been running awhile and it had a th350. At that time the gauges all worked as with the interior lights and gauge lighting all outside lights came on except the fender markers. Turnsignals wouldn't work right either. I replaced the flasher and all bulbs a couple of times to eliminate having bad new bulbs still having problems.
After I did the t56 swap on the first drive (at night) I did a small clutch drop and all the gauge lights and gauges died. I limped it home but all exterior lights were still on. I had the cluster just sitting in at the time not bolted up since I had been experimenting with changing the resistors on the fuel gauge to make it work with the 4th gen tank so I figured that was the issue. I got it all bolted up and have driven it 500ish miles or so the last 6 months or so and the turn signals would randomly work and not work and not always right.
I recently installed my 4th gen headunit and have it all wired in with the newer pigtail so it's all plug and play. I've had the car 10 years and I've never messed with any of the body or dash wiring other than the engine harness btw. Now what it's doing I start the car or switch on and both signal indicators light up radio tries to start up with cd and all the interior lights start flashing with the radio and now the tach is the only thing working and even then it's hit or miss. I can turn the headlights on and all gauges die. I can roll the dimmer switch up and play with it to a certain spot and the gauges and gauge lights will come back on but it's touchy. I've also replaced the dimmer and headlight switch with a known good one and the same thing happens. The pass side fender signal is the ONLY exterior light that does not come on at all no mater what. All the others will at least come on in one of the many combinations I have tried lol. Now the only time the gauges radio and gauge lights will work is with the door open. I shut the door and they turn off. Found out today if the car is running with the headlights on and I turn off the switch the car stays running. I did notice before I could have the car off and key in my hand turn on the headlights and hit one of the signals and the fuel pump would run.
The only known issues I've noticed with my swap harness is I have no wiring for my hvac resistor and the ses light stays on no matter what. Other wise the car runs and drives great. If I cannot find a easy solution soon I'll be pulling the front clip and part of the interior and start going through the body harness and checking and re wrapping it. Hopefully it's a simple fix. Here Are a couple videos I've put on YouTube to show the issues.

 

 

Since it's intermittent I'd wager on a loose connection/ground

I'd start by pulling the cluster and checking power and grd on the cluster connectors. Then activate the different light circuits and see if there's power or ground missing

Are there any codes in the PCM? If not and the MIL stays on constantly, everything might be trying to ground through that through the PCM

 

I've had the cluster out and swapped in my spare one a few times and it still does it. I keep getting a code for the vss and I don't know why. All the wiring is correct as far as I can tell. If I clear the code the light still stays on. However if I just let it go and don't clear it after awhile it just turns off till I go to clear the code again then it comes back on. I've been meaning to get out and check the marker wiring but between the weather and work I haven't had much of a chance. It just seems like things have progressively gotten worse. The car runs and drives great though other than sometimes stumbling when coming to a stop because of the lack of a vss signal.

 

Stumble at stop means the VSS signal isn't getting from the sensor to PCM. Do a continuity test on the two wires

Retest the continuity of the cluster grd. Just a hunch

 

Just set the multimeter on 12v? What is the best way to check the connections? I'm a novice at wiring lol.