Before you waste a lotta time, Check the rear ground strap. It's in the trunk close to the latch.
Check it for continuity to the chassis.
Then check from the chassis to the sockets and make sure ALL the bulbs are correct and installed correctly, and all the bulbs work as designed.
All means rear and front.
After that
if the problem still has not appeared, I think I'd be taking the signal switch apart, and digging the contacts out of the melted nylon channels.