The other day I started having problems with my tailights. With the parking lights on stepping on the brake pedal caused the right side lights to out, the left side to dim and the back up lights to come on dim.
Generally a problem like this indicates a ground problem so I got out my GM Shop manual and started looking through the schematics. After scrutinizing the schematic I found that there is only one ground for all the lighting at the tail of the car. That ground is located in the back left corner of the trunk right under the connector that mates the body wiring to the tailight wiring. Along with this there are several splices in the harness where the grounds branch out.
To be certain the problem was in the taillight harness I unplugged it from the body harness in the trunk and used a spare battery to test each lighting circuit and indeed found that the problem was isolated in the tail harness.
I found it easiest to remove the tail harness from the car, After removing the tailight bezels, license light housing and side marker connectors it took about 3 minutes to remove the harness from the car.
I pulled the wires from the tubing cover and found that there are several splices in the harness. Two were for the ground circuit and one was for the parking light circuit. After removing the tape covering the splices I found that the splices were simply uninsulated barrel crimps and one of the ground splices was severely corroded. Since I had it apart I repaired all three of them by cutting the wires back until I reached good un-corroded wires. Then used barrel splices for the repair and soldered each splice. I then covered the splices with sealing heatshrink to prevent any further corrosion.
I tested each of the circuits again and everything checked out ok so I re-installed the harness. Once the harness was in I tested the circuits one more time and  wouldn't you know it I had another problem, this time on the left side of the harness.
I removed the harness once more and found one more small taped area that I had missed on the first go around because it was a small thin section on one single conductor. What I found under the tape was the contacts from a connector being used to join the two ends of wire together and when I took the tape off they fell apart in my hand.
No wonder GM had such a bad reputation for mid 80's cars. Some union assembly line worker was just too lazy to get a longer piece of wire or to at least splice it properly.

After repairing the last splice everything once again checked good and all my lights are functioning properly.

I took the time to write this so that others would be aeare of the potential problems in this harness and it is easy enough to repair that I would recommend taking an afternoon and performing the repairs to help prevent any problems if you do not already have them.