got one that is pretty elusive. 1992 E-350 RV chassis. Factory harness extends to behind rear axle, RV builder from there on. RV wiring pretty MM to look at, but it works.
No power in brown lead at end of harness. Problem started w/ blown fuse, new fuse fixed it for a while, then no lights & fuse still good. Replaced headlight switch, no cure. Original switch looked good ( no corrision ). To save my vacation, ran new wire to end of harness and plugged it into cigar lighter. Crude, but it go me by.
Traced every inch of harness down frame rail ( one ) no sign of stress or wear on plactic conduit. Actually, wiring looks pretty new like for a 15 year old truck. Have 460 CID engine w/ auto/overdrive. Runs good, but only get about 10MPG.
Been fixing electrical problems in trucks since 1961, but usually had something obvious to go on. This one has me stumped. Is the firewall connector plugs a possible source of problems? This truck has 2, and everything is well bundled before getting to them, so checking individual wires is not possible.
Bought the manual for RV chassis, no help, no scematic or wiring color code.

Got the answer to a tricky speedo problem on my BMW K1200LT from a rider forum, hope this one can help w/ thes tail lights.

john

 

Odds are this is wired thru the multifunction switch in the steering column. Have you checked that?

 

Only the rear turnsignals and brake lights are wired through the multi-function switch.

Do the front marker lights work? The brown wire is fed from the headlight switch, and goes all over the truck to all the marker lights front and rear.