Join Date: Feb 2011
Location: San Francisco Bay area
Thanked 83 Times in 82 Posts
Rep Power: 7
What you need to do is test the continuity of the ground. You need a multimeter for this, on ohms range.
Check that you have around 0 ohms (might be 1 ohm, no higher) between the vehicle ground and that pin on the car's 4 way connector.
Then check between the trailer's ground pin and anywhere on the trailer chassis where you can touch exposed metal.
There should be a wire from the trailer's ground pin, to the trailer's chassis. If this is broken (can be broken internally so it looks OK) or has a bad connection this will stop all the lights working.
These things age quickly from regular dipping in water!
There's only so far you can go with a test light. A multimeter is a much more powerful tool.
Bristol 31.1, San Francisco Bay