Yes, I have run into this problem with my former '88 C1500, and here is what was at issue for me.
If you take out either rear light assembly and remove the light bulb holder base from the lens, take a close look at each bulb socket very carefully. You will notice that the metal ring the bulb seats into to make contact is actually only press fit into the plastic socket. The strip of metal from the ring that runs down the side of each socket makes contact with the main circuit and maintains contact by using pressure only. What happened with mine was the point at which both small pieces of metal made contact got dirty enough over time to lose continuity, although they still appeared to be touching. All I had to do was shine up the connection point with a small wire brush and add a drop of solder to make permanent contact again and the lights worked like a charm after that.
Hope this helps.
Cheers - Jonathan