the ground is via the case of the socket and the body of the light, no "ground wires" on anything, just make sure the body of the light is well attached to the truck, which is the ground - sounds like what you done is set things up so under some conditions, the power to the bulb is being shorted to ground :confused:

Bill

Moved over to the Passing Lane

"When we tug a single thing in nature, we find it attached to the rest of the world" ~ John Muir
"When we tug a single thing on an old truck, we find it falls off" ~ me
Some TF series details & TF heater pics