Thank you for the help with hooking up my truck cap lights (third brake light and dome light). In your description of the four prong trailer plug you never said what the "d" wire is for. Just curious!

Thanks!


"i would recomend a 4 prong trailer plug.

wire a would be for the hot side for the 3rd brake light.
wire b would be power for the interior light.
wire c for a decent ground.
wire d explained later"