Checking out the rear harness is easy. Get your service manual. Find the section showing that diagram. It's a simple harness with the kick panel plug on one end, and the tail lights on the other Basically this is what you have in that harness
1.....Maybe power for the trunk light circuit (This might be an option)
2......The wire to the fuel tank sender
3.....The wire for tail lamp / marker power
4.....The wire for left side brake / turn
5.....The wire for right side brake / turn.
The trouble with only one lamp could be right at the socket or right in the bulb.