I just installed a Sony Deck(GTX500 I think) and now I have no rear door speakers..
I used the plug in adaptor from the new deck to the stock harness for the rectangle plug, but I read that the second (square) stock plug on the back of the stock radio was just unused now, might some connections have been in there?
Thanks for any help!

 

Doesnt do anything. Check your wiring first. Make sure all pins are in the connector, then check the outputs on the radio.

 

Agreed. Also make sure your balance/fade, can't remember which, is set properly, so that it is not all in the front.

 

fade=front to back
balance=left to right

x2 on what Low said. Also, check your connections between the radio harness and the truck harness.