HI EVERYONE... I BOUGHT A 1996 E350 7.5L FULL SIZE CARGO VAN A FEW YEARS AGO. I AM NOW JUST PUTTING IT TOGETHER SO NEVER NOTICED THAT THE REAR TOP BRAKE LIGHT OR TRI LIGHT WAS NOT WORKING AND THE BOTH INTERIOR DOME LIGHTS WERE NOT WORKING. I REPLACED ALL BULBS BUT STILL DID NOT LIGHT. FUNNY THING HAPPENED WHEN I STEPPED ON THE BRAKE PEDDLE. THE DOME LIGHTS BOTH CAME ON?? THERE IS A SHORT SOMEWHERE SO I AM IN NEED OF A WIRING DIAGRAM..CAN SOMEONE HELP..THANKS

 

Try tracing the wires from the bulbs. Chances are it's under the dash. Also another crossed bulb or a misplaced fuse could cause this. When a dual filament bulb blows one filament and it crosses the remainder of that filament to the other electrical backflow can occur.

 

Another easy way out if you think there's a short - sniff around for burnt plastic (wire casing).