I have a 2015 gmc Sierra P/U. I noticed water was getting in my rear right tail light. It burned out one bulb. I replaced them all.Problem fixed.
Several months later my tail light wasn't coming on with running lights. I replaced it with a OEM tail light and new harness. I cleaned out the junction box with rags and it (contacts) appeared clean. Connected the new harness. No lights for running but have brake and turn signal. Any ideas?

I would load test the feed line. If your connections are good you have a resistance in the wiring.