Odd brake/tail light problem

Here's where I'd start. I assume, is it? a factory harness?

Disconnect the rear harness at the left kick connector. Get a diagram (You can download wiring diagrams and or service manuals from MyMopar) "Rig" a alligator clip to power THROUGH A TAIL STOP LAMP in series so that it will limit the current.

Now apply power to each of the rear lamp terminals and verify they work. Tail, left, right, and reverse

If that all works OK, now take your "test" stop/ tail lamp and ground the end you had hooked to battery and use it for a test lamp. This time, access the lamp pins in the front kick panel connector and see if the output power is what you'd expect, in other words tail, stop, left, right turn.