Using an M12 battery to test out a trailer rewire. A lot easier than having to pull the truck in just to test a couple lights. So handy! : r/MilwaukeeTool
How to quickly and easily test trailer lights with a battery - YouTube
Wiring Trailer Lights with a 7-Way Plug (It's Easier Than You Think) | etrailer.com
How to Use a Circuit Tester
Recommended Way to Test Trailer Lighting With a Car Battery to a 4-Way Trailer Connector | etrailer.com