I had a similar problem on a trip a few years ago.
What it ended up being was a wire along the frame rail had worked its way out of the harness sheathing, and had become pinched between the frame and the camper box, causing the lights to short out.
It happened on and off over the course of the week, and was a bear to track down.
I eventually re-wrapped and re-routed the harness to prevent a recurrence.
Hope that helps
I came across this thread via google. We have a 95 Fuso Canter here at work that I'm trying to track down a brake light issue on. I'm finding it next to impossible to find information online for them. I've got rear turn signals and running lights but no brake lights. Do these have a brake light switch up near the brake pedal or something down on the air/hydraulics for the brakes outside the truck? If someone still has the wiring diagrams handy for them I'd appreciate it.
Thanks I'll look up in there closer tomorrow. I couldn't see anything in there resembling a switch but I didn't really know where to look.