Awhile back I noticed that the passenger rear speaker was not working. I decided to try to mess with it this past weekend. I know that it is not the speaker because I swapped the rears and the problem was still there. The connections behind the radio seem fine.

What else could it be and how do I fix it?

 

perhaps check the connector to the stock amp in the passenger foot well?? ... it might be worth checking whether you hooked up the replacement speaker correctly as well