I found an old thread that would not let me reply to so I will add this one as I think it will be useful to someone. The old threads help me. Thanks!!
I spent the day wiring a 7-way trailer connector to my brothers 2002 Ford E450 chassis (with a 2003 Gulf Stream Coach body).
The factory brake controller connector was near the emergency brake pedal, taped to wires on the far left. There were two similar connectors. Both would plug into the brake controller wiring harness. The one with 6 wires was totally wrong. Use the one with 4 wires, one of which is a large blue wire for trailer brakes.
Next was locating the blue wire under the chassis. It ran along the driver's side frame and ended just behind the rear axle/in front of the gas tank.
There were three wires coming from the front:
1. Blue for trailer brakes.
2. Orange for B+
3. Black w/green tracer for backup lights.
Next to it was another connector with all the wires for the running lights, left/right turn, and backup lights.
The Black/green (backup lights) was the only one that passed on back. An orange wire connected it to the group of wires of the other lights mentioned just above.
The existing plug only had pins for the one orange (backup lights) so I had to cut off the plug and receptacle. I then soldered on 13 feet of wire as follows:
Blue to #12 blue for trailer brakes
Orange to #12 red for B+
Black/green to #18 orange and reconnected the existing orange wire.
The relay for the B+ was missing from the fuse/relay box under the hood. I got one out of a junk car. It is the one at the back/center, just behind another relay slot that had no pins for connections in it.
All worked very well.
Hope this helps someone as earlier posts help me.