The "P" bodies were known for sub standard wiring issues throughout the car. The ground wires are always at fault.
I have owned several of these cars and have had to re-do everyone of the rear pigtail sockets for the tails, turnsignal, and backup lights.
Do your research on line and you can normally order the exact ones you need.
is where I went. I was able to get the exact ones I needed.
There are so many options, so be sure you order the correct ones.