The typical problem here is that the sockets are not grounded. Sometimes bulbs can be cross shorted between filaments internally and cause interesting problems.
One easy way to test, other than just "wiggle testing" the sockets, is to turn the tail lights on, and jab your test lamp or meter onto the socket and to a good trunk "body" ground point. Voltage there shows the socket is not grounding
You can solder a separate ground pigtail direct to the socket shell if you have a big enough iron (pull out the pigtail and spring first) and bond that to ground. The "easy" way out is to clamp a wire around the socket with a hose clamp
