I solder a dedicated ground wire to each of my tail light sockets that gets grounded at one end just so I I don't have any strange problems like this. Remember, electrons flow negative to positive so the source is actually the frame and body.
Brad, have you tried wiring everything up on a bench with a battery and bulbs to see if the same problem is there? Have you tried a good ground on all of your sockets? Just using wire and alligator clips will work for testing.