Generally speaking the brake light is the directional light and the selection of operation is made in or close to the steering column. Now having said that have someone at the back of the truck and turn the brakes on and then the directional and make sure they are the same bulb on the LEFT side. It may be a case where they are using 2 different bulbs for brakes and directional. If that is the case check the right side directional bulb first for the problem and if not that look at the socket and ground connection for that bulb.
If the bulb is the same for the directional and brake light the problem may be in the directional switch itself. You would have to have a schematic of the switch to check that out. That would come from the chassis manufacture and should be easy to get either at the library or online
WBCCI 12156 AIR 3144 WACHUNG TAC NJ6
2004 Excursion 4x4
1991 34 ft. Excella +220,000 miles, new laminated flooring, new upholstery, new 3200 lbs axles