Step 5: Splice in the Trailer Wiring
Use a couple feet of four-wire flat ribbon wiring harness, with snap-fit splicing connectors.
Splice the brown wire of the trailer ribbon to the solid brown wire of the truck wiring.
Use pliers to force the metal tab into the connection, electrically joining the two wires.
Finish the splice by snapping the plastic tab in place.
Now, splice the same brown wire of the trailer ribbon to the white-stripped brown wire of the truck wiring. This connects both left and right taillights to the one brown wire of the trailer harness.
Splice the yellow wire of the trailer harness to the yellow wire of the truck wiring.
The ground wire screw attached to the steel chassis is also a good place to clip the electrical probe for testing.
Splice the green wire of the trailer harness to the dark green wire of the truck wiring.
Splice the white wire of the trailer harness to the black wire connected directly to the steel frame of the truck chassis. This is the ground or common connection for the entire electrical system.