I bought a trailor brake control moule for my ford expedition 2003 but can't see where to plug it in.

The factory conector is located under the access panel under the dash on the drivers side to the right side of the steering column attached with a plastic clip to the backside of one of the dash braces. Im assuming you have the factory supplied pigtail for connection to the factory wiring, also you will need to add a relay to your distrubition block under the hood, there is an empty location for the trailer brake power relay, your controller will not work if you don't install the relay.