89 E350 No right brake light

I've got an E350 cutaway (bus) with a right brake light that has stopped working. All the other lights work properly. We've found the dead wire and traced it as far as we could. It gets lost once it enters the front of the vehicle. I suspect the problem is wherever the left and right sides branch off after the brake switch but I don't know where to look for that. Schematics from Chilton and Haynes are of no value, they don't show much either. Any help would be greatly appreciated as we haul kids back and forth to church in this vehicle and want them to be safe.