Check the wiring connections on the antenna. If you have the antenna on the rear door it may not be getting a very good ground too. The body of the truck acts as the ground plane for the antenna so you need a solid connection to the main part of the truck. You could try running a wire between the door and the rest of the body and see if that changes it. There should be something you can hook to on both sides to test, and if it helps you could hide a short wire right by the hinges.

"Time and tide melts the snowman"