Abstract: A portable apparatus for checking the wiring orientation at electrical connectors of trailers or trailer towing vehicles is described. The housing of the apparatus supports a rotary switch and a plurality of multi-terminal connector plugs and/or receptacle portions. The rotary switch has a plurality of switch positions with indicia at each switch position indicative of a particular electrical function such as a particular lighting system in the trailer or trailer towing vehicle. Each switch position of the rotary switch is connected to each terminal positioned on each connector portion at a location that is industrially standardized to electrically couple an electrical system in the trailer towing vehicle to a corresponding electrical system in the trailer. By moving the rotary switch from position to position the orientation of the wiring at the connector on the trailer or trailer towing vehicle is checked by sequentially activating the electrical systems in the trailer or trailer towing vehicle.