My trailer lights and brake controller recently stopped working and I had to troubleshoot the problem. In this video, I show how to test the 7 pin trailer connector with a typical Multimeter. Turns out it was a problem with the trailer wiring and connector.

My hands are in the way so I displayed a diagram in the upper right of the video that shows the the location of each pin to test the trailer connector. To test, refer to the diagram and the following instructions:
1) Turn on vehicle ignition.
2) Go to trailer connector with your multimeter and set it to measure DC volts. You may want to test it on a random battery to make sure it's in the correct mode and working properly.
3) To test the trailer connector, put the black probe on "-12VDC Ground", according to the diagram
4) Put the red probe on the circuit that you are testing. For example, to test the trailer running lights, put the red probe on "Running Lights", according to the diagram. For things like brakes lights, you may need someone in the car to apply the brakes while you measure voltage.
5) You should get voltage between 11-14 DC volts (depends on condition of battery and whether is is being charged/drained)