I just bought a 97 Dodge Ram 1500. The Tail lights work when I just have running lights on, but don’t work when I pull the light switch out far enough for the headlights to come on. Also, and probably related, the dashboard and radio lights don’t work at all. I replaced the switch assembly already and that didn’t fix the problem (i didn’t use the supplied wiring though, only the switch).
The previous owner used it to pull his boat so the rear end may have been backed into the water. The brake and reverse lights.
If there was a trailer hitch there is a good chance that the wiring to the trailer connection is the cause of your problem.
It has both trailer light hook up and trailer brake plug. I will start looking for ground problems there. Thanks.
That last sentence of my O.P. should have read ‘The brake and reverse lights both work’.