1991 Chrysler Imperial Tail running lights won't work.

Hi OzzmanFloyd120. Welcome to the forum. Chances are you overlooked a blown fuse for the tail lamps / running lamps. The most common cause is using the wrong wire for the ground wire for your new radio. There is no ground wire in the two 7-pin plugs for the original radio. Two wires measure like ground wires because they measure very low resistance through all of the bulbs. One wire is tied to the tail lights to tell the display to dim when the lights are turned on. The other wire is tied to the dash lights to tell the display how much to dim.

When you connect either of those wires to your radio's ground wire, the circuit becomes grounded through the outer shield of the antenna cable. That's what causes the fuse to blow.

caradiodoc

Thanks for the welcome, I'll take another look.
I'm just wondering also if it's possible that the aftermarket wiring harness? It just seems that it would be designed to avoid this.
Also, if it is a flaw in the harness' design, what should I do so I can use both the new radio and the rear running lamps?

Oh. You didn't mention the adapter harness. The problem comes from people who splice the new radio's harness into the car's harness without using a wiring adapter. I never looked at those adapters so I don't know how they handle the ground. The original radio had a braided strap bolted to the radio's case. Later in the '90s they went to a light black wire that was clipped to a terminal on the back of the radio. If you don't use that ground wire, your new radio will be grounded through the outer shield of the antenna cable. That shield is very effective at keeping out interference but it makes a very poor power ground for the radio. Use the original ground wire for your new radio.

There should be two places to look for fuses if I remember correctly. My Dynasty is a '93; there are some differences between the two model years, but most of the electrical wiring is the same. The fuses inside are behind a removable plastic cover inside the glove box. By '93 there was a second fuse box near the battery. I think your '91 has that too.

If you look closely at the fuses, you'll see they have two tiny holes on top. Use a test light with a sharp point to touch those points on each fuse. Turn the head light switch on so there will be voltage to the running light fuse, then test both terminals of every fuse. You're looking for one that has voltage on one terminal but not its other one. That's the fastest way to find a blown fuse.

caradiodoc

Got out the multimeter, no power on the harness where the bulb plugs in, replaced every fuse, tore the dash apart again, took out the stereo, took a look at the headlight switch and it looked fine (at least It looked fine to me, I don't know what it would look like good or bad, as it was the first time I ever pulled it out and looked at it.)
There's probably a short in the harness somewhere... I have no idea. I'm probably going to buy my friend's Grand Am and let my Imperial sit in my grandparents back yard until I have the money/time/know-how to fix it or have it fixed.

I appreciate all the good advice tho.