Turn signal

Get a test light if you don't already have one. Out the clip to ground, makes sure it's a good clean ground, pull the bulb sockets out of both tail lights, have someone step on the brake, with your ground connected make sure the right brake light is on, remove the bulb and probe the terminal, it will light your tester if your ground is good. Move to the LEFT light and do the same thing using the same grounding point. If the tester lights the bulb is bad, if not go to the plug on the harness, unplub iit and see if it lights there, from there you can move to the signal switch if you still have no light. I think the left rear is a yellow wire at the switch, but check a diagram to be sure.