Hi: Thank you for coming to Just Answer with your question. My name is Randall and perhaps I can assist you with this issue.
Start with the connections which are usually behind the left hand taillight assy. It can be the feed on the positive side or the ground. Take a 12 VDC test light and hook it up to a good ground and see if you have a good 12 VDC feed to each one. Take the lens cover off and the bulbs out to test the socket. This can tell you if it is the ground side or the feed side. You should be able to track it down this way. Sometimes the ground side can be best repaired by running a new ground lead. Please get back to me with what you find and we can go from there. R.J.