[ImperialCrown]

Bad socket grounds. We used to replace the brass ground terminal in the sockets instead of replacing the whole socket. You may be able to spot solder a ground wire to the outer socket shell and splice it to the existing black ground wire for a good connection.

 

[ImperialCrown]

It's the grounds at the socket bulb holders themselves. If you jump a ground wire to the socket shell, you should notice a brighter bulb and a return to flashing turn signals.