Since the brake light is missing from both the truck and trailer, it's not very likely (but still possible) that burned out bulbs are the problem.
There are no fuses that can affect only one rear lamp. Sorry.
The single brake lamp signal from the pedal switch goes to the multi-function switch MFS. The MFS splits the brake signal into front and rear and left and right signals. From there, it's a straight shot to the rear of the truck for the towing adapter and the actual brake lights.
I'd suggest inspecting, cleaning, and testing the MFS switch. Its connector pins and the harness have cause problems on a lot of different Fords. If the connector is in good shape, the MFS is probably defective. The service manual gives specific point-to-point checks to test it before replacement.
Last edited by projectSHO89; 02-08-2006 at 10:16 AM.