The tail lights and license plate lights are not working but the rear turn signals, back-up and brake lights work fine. The front parking and headlights work fine. None of the fuses are blown and testing at the rear coupler, the yellow tail light wire that comes from the firewall has no power. My questions are: 1] could this be a relay 2] where is it located? 3] how do you tell if a relay is bad?

I have a '92 chevy Suburban 1500 and I pulled a trailer the other week, only to discover a couple weeks later I had this problem. I'm baffaled that all my lights including the front lights are fine, but only the two rear tail lights are out. Any thoughts would be appreciated.