Ok so here's what I got
I have a 1988 firebird formula that the tail light harness has been hacked up. I replaced with an 88 GTA tail light harness.
I have not modified the under dash wiring with the blue wire off the brake light switch where you tie it in to the white wire on the steering column.
When I check the lights just going with plug and play, I have brake lights, reverse lights, left and right turn signals and emergency flashers
I do not have parking lights. No lights come on at all when I hit the switch on the dash. Is there something I'm missing?

​​​​

 

Check the TAIL fuse.

 

Tail fuse is good. Even checked for power on both sides of the harness plug which there is 12 volts on both sides with the dash switch on according to my test light. I even hooked up the rear plastic panel with it's set of plugs that partly share with the tail light harness and tried it with still no lights out back. This GTA harness has an extra small plug which I think may have been for the GTA only diagnostic indicator on the dash but I don't think it has anything to do with this problem. Not sure what else to check.

 

I finally figured it out. The small plug in the harness does effect the parking lights. I had to take the purple and brownish/black wire and tie them in with the switched 12v for the parking light switch in the harness which is the brown wire almost center of the harness. One is the left side and the other is the right side parking lights. The 2 white wires when you cut back the factory tape are just looped into the plug. The orange wire on the small plug unless it's a GTA with the DIC digital dash doesn't get used in this case,, It's for when you pop the hatch it tells you it's open.
I'm sure the lights ran thru that plug for the GTA DIC also for when you hit the diagnostic button on the digital dash it also did a light check and sent a signal to all the lights to let you know they were working.