I was wondering how to splice the wires to hook up a four prong trailer light plug on a 94 f-150.

 

If you do not have a towing package you can buy a adapter that connects inline with the harness that has a 4 pin plug on it. If you have the trailer tow package there is a plug or plugs on the drivers side of the frame rail by the bumper mounts that should be in the standard 4 pin plug color codes. On my 93 there was a 4 and 3 wire plug the 4 wire controlled the lights and the 3 pin was Trailer brakes, 12 volt keyed power and backup lights.

 

If funds are not available for the adapter as Mitch mentioned, this is the universal trailer code. Yellow wire is Left turn and brake. Green wire is right turn and brake. Brown is running lights. White is ground.