[f5fstop]

At the rear:
BROWN - Trailer Park Lamps Supply Voltage - runs to Fuse 10 (TRLR) and then to the PRK/LP Relay
YELLOW - Left Rear Stop/Turn Lamp Voltage - spliced into left rear stop/turn lamp wire
DARK GREEN - Right Rear Stop/Turn Lamp Voltage - spliced into right rear stop/turn lamp wire
RED - battery positive - runs to Fuse 84 (TRLR B+)
LIGHT GREEN - Backup Lamp Voltage - to backup lamp circuit
DARK BLUE - Auxiliary Supply Voltage - runs to Dark Blue wire under dash that is part of the three wires above the brake/accelerator pedal (guess this is the signal to the trailer brake?)
UNDER THE DASH:
DARK BLUE - Auxiliary Supply Voltage (See above)
RED - Runs to fuse 83 (E-BRAKE)
LIGHT BLUE - Spliced into stop lamp circuit.

Not being familiar with trailer brakes, I would guess the following:
The RED wire under the dash supplies voltage to the brake unit, and applies a relay via the DARK BLUE wire that runs from under the dash to the rear. The LIGHT BLUE wire I would guess is also some type of signal wire to the brake unit to signal the unit the brakes have been applied. Again, I know about as much about trailer brakes as I do warp engines, but it seems logical.