1993 E350 Ambulance - tail lights / Brake Lights issue

[Matt M.]

A long shot, but I'm desperate...

I am working on an out-of-town friend's 1993 E350 Ambulance (decommissioned) that when the headlights are on I have no tail lights or brake lights. This holds true even when the parking lights are on. When the headlights are off the brake lights do work.

I have checked all of the fuses under the dash - all good. My next guess is the replacing the headlight switch in the dash. I'm assuming there's another much larger fuse box located somewhere in the vehicle, but the fact the brake lights work when the headlights are off leads me to believe this is not a fuse issue.

If anyone can provide some guidance or lead me to a different resource for posting this question I would very greatly appreciate it.

Thanks

[Matt M.]

10/12/2012 - Update

I installed a new headlight switch and at first I thought the issue was resolved. I had full taillights and brake lights (upper and lower sets of rear lights) while the parking lights were turned on. However as soon as I turned the headlights on the rear went completely dark again - no taillights, no brake lights. Shut the lights off completely and only the lower set of brake lights work. It doesn't make sense.

I pulled the new switch off and sprayed the inside of the harness with electrical contact cleaner. Reinstalled switch - no change. Initially all lights work in the back until the headlights are turned on and then I lose everything.

Does this sound like a grounding issue? I believe my issue is under the dash, I'm not under the impression its related to the back of the vehicle (ambulance cabin).

Any thoughts or suggestions - I'd greatly appreciate it.

Thanks