So the problem is in the right rear signal light assembly? Take it out again, turn on the signal, and gently move the wires around (to simulate installation) until it shorts. The problem is probably in or near the socket. If not, follow the power wire back into the body as far as you can and look for broken insulation. If not, encase the power wire inside a piece of flex tubing or wrap it in electrical tape.
Electrical problems like this can try your patience. Good luck!