I'm going through the same with my 03 reg. cab. I'll try to trace the wires and let you guys know what I find. Kinda stupid that they didn't rout the wires
... but at least they gave us an install friendly rear trim panel.
EDIT: So I pulled the drivers seat and the door side trim and rolled the mat back... the connector is smack dab under your *** in the 2003 model, it is plugged into the harness for the 3rd(cab top) brake light I just snipped stripped and soldered then wrapped some speaker wire that I ran to the rear locations under the mat and rear trim panel. Fairly easy install once you know all you need to do is pull the seat and that trim panel at the bottom of the door.
Where I screwed up was that I bothered to pull the stickshift and all the trim around it thinking I would need access there, you really don't need to do so.