1995 Chevy k1500 tail light issues

The problem might be with the multi-function switch.

https://www.rockauto.com/en/moreinfo.php?pk=929397&cc=1051119&jsn=528

All the exterior lighting circuits pass thru this switch.

These switches can wear where when you operate one circuit in the switch it can have negative effects on other circuits that pass thru the switch.

Tester