How to test 4 pin electrical on trailer?

The first thing you need to understand is that the four pins work the ground (the exposed one) the left turn the right turn and the clearance lights. You will notice I did not say brake lights and this could be a problem for you. On older cars the brake lights were the turn signal bulbs on newer cars the turn signal are a separate lights. If your is newer and the brake lights are separate you may be able to get a adapter at a trailer place. If yours is not separate you only need a power supply (your cars plug) and a test light. Make the ground connection to the trailer and put on the parking lights and plug a wire into the plug that is hot and plug it into the trailer plug until you get the right lights to come on.

It's a pain in the butt trying to test the circuits without the towing vehicle hooked up to the trailer. I'd recommend getting the vehicle hooked up and testing it that way first. You need an ohm meter to check for continuity if no vehicle. Get a long wire with alligator clips and hook up the one side to the plug on the trailer and the other end to the meter. Then probe each light contact to see if there is continuity. You also need to check the ground terminal for continuity to the frame of the trailer or the light housing mount. That still does not mean they will work. Who knows if the trailer wiring is routed to the correct harness pins.

With a $10 test light from the auto parts store.