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.
www.wiringproducts.com 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.