first off, check and see if they are properly fitted to the tail light holder and that they have clean and good contacts..
then if its a manual, it is mosk likely that your reverse trasnmission swicth is about to fail, ,mine is gone, just waitng warmer weather to change.
anyway to replace, lift the car, get a new swicth and jsut uncrew the other and put the new one in. and then plug the wire to it.
if its an automatic, then the gear selector sensor is gone.
there is a electric siwch underneat the gear selector that tells the computer what is the selected gear.. and it also turns on the reverse lights.
any questions. ask.
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