fuse first and for most. umm also check the sockets to make sure they work and make sure the bulbs are touching both things in the bottom of the socket.
Do you have the original tail lights? They will corrode. Take some emory cloth and clean inside and out. You need to do where the bulb goes plus open the trunk, pull the harness off and clean.
If your light bulb socket is loose it could be a bad ground. If your wires are original they are probably brittle and since they run exposed in the trunk I would look for damage to the harness.
But before you do anything........grab a volt meter and check for voltage at the switch. If ok then locate the harness on the left side of your trunk (near the rear seat back and check again. The left light controls the right.
This should help you isolate the area to look for a problem.
ok. you wont believe what it was.. after 4 hours of me diagnosing it my self.. i call in a professional.. a buddy of mine is an electrical engineer.. he does the same trouble shooting i do and more.. he notices that the yellow wire.. i believe 10 on the schematics is weak and pulsing power.. we unplug the connector and plug it back it.. it works fine..