A 91 still has the old fashioned lighting system, it wasn't until after 2000 sometime when the lighting computers came along.
As a side note, if someone cobbled in a trailer hitch plug connection to your truck, I would start by checking out the wiring for that trailer hitch plug. those trailer hitch plugs are often the cause of tail light problems. Those quick connect blue things most use to connect the hitch plug to the truck often break or badly corrode the truck harness wires.
The tail light (and plate light) function comes off of the headlight switch, both tail lights come off the same wire terminal on the headlight switch. That wire generally travels along the frame to the splice where it splits, one wire enters the left tail light 4 wire terminal, the other side of the split travels across the rear frame crossmember where it again splits, with 2 wires leaving the splice to go to the license plate lights (a right and a left light), and the main wire continues to the right side tail light pig tail.
A 91 should have the specific ground spade on each bulb socket in the tail lights that would connect to a single terminal in the square taillight plug on the light side. I believe all the grounds on the frame side are still grounded to a single contact point on the frame, but I don't remember where that is.
The square pigtail plugs tend to have a few issues. They have 3 female terminals and one male terminal on the frame side and 3 male and one female on the light side. for each side of the truck. The odd terminal is the tail light ground terminal. One of the three of the like terminals would be the tail light terminal, one terminal would be turn/brake, and the 3rd terminal would be for the backup light. It is not unusual for any number of those male terminal posts to corrode and break off.
The stop/rear turn light power travels through the stop light switch at the brake pedal, then that power goes into the turn signal switch. Its at the turn switch where the brake light is separated from the rear turn. Each of the 4 turn signal power wires has its own power source from the turn signal switch. One power for each front turn signal (the dash indicator lights are powered through both front turn lights), and one powered for each rear turn signal.
I think on your 91, after checking the hitch plug connection, I would start the the search for the issue by looking at each tail light square plug. pull the plug apart and inspect the condition of all the terminal ends. It that is good, you can test the ground to a good ground source with a continuity tester. With a test light you can see if you have light power at those terminals too. Gene
