Tail lights
2 Answers
Wirings bad, broken, rust, bad socket, bad bulb... Few solutions, test with a mutimeter, or just buy a new bulb socket and some wire and splice into the other side, if they are brake lights and not turn signals they both come on together regardless.
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http://www.yukontaillightrepair.com/about/ You may find some useful info there. It's a common problem with the newer GMC Yukon's. http://www.yukontaillightrepair.com