Grounds. Each light has to have a direct path to ground. If not it will find a path to ground through the other filament in the bulb, up the tail light wire to the other tail light, through both filaments of that bulb. Ground each light housing. Also be sure the bed is grounded to the frame and the frame is grounded.

Did I mention you need good grounds?


"It's just a phase. He'll grow out of it." Mama, 1964

1956 Chevy 1/2-ton 3100
1953 Chevy 6100 "The Yard dog"
1954 GMC Suburban Now with a new proud owner.