Bad ground? Bad connection inside the socket? Broken wire?
Noboday can really tell you why your tail lights don't work, without seeing
the vehicle. You're best advised to start with the basics and work your way
through fundamental troubleshooting. Do look first and carefully at the
grounds. Automotive wiring will produce some very bizzare symptoms from bad
grounds. Components will see something that sorta looks like ground feeding
back through other components and produce completely unexpected results.
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