Why won't passengerf tail light doesn't work?
ext set meter to ohms and with the bulb out attach ground lead to frame or other grounding point on the truck, and with the other lead test both contacts in the bulb recepticle, on one of them you should read continuity (low resistance). If not check your wiring from the socket to ground. the next test set your multimeter to dc volts, turn on the lights to where this light should light up Take probe and measure the center contact (should read approx. 12-15 volts). If not check your connections on your wiring harness. It sounds like the voltage is not making it to the bulb. double check your harness and if possible take a voltage measurement at the harness. Chances are that theres a bad connection at the harness.
Search GMC Yukon Questions
GMC Yukon Experts