Currently I have had a friend towing me and my trailer to the water while I save up for a hitch for my car. On the last trip, the trailer lights didn't work. I have been given a Multimeter. Can I use this to test the flat - 4 pin connection? How do I use this thing?
The flat pin connector has 3 exposed pins and one covered. The covered pin is the ground. You can use the multimeter to test continuity between the other 3 pins and ground. Set the multimeter in ohm mode. Put one of the leads in the ground pin and the other lead in any of the other pins. The reading will not go to zero because of the resistance in the wire. If (on a digital multimeter) the reading stays at one, there is a disconnect somewhere. If all 3 exposed pins read the same, you have a bad ground connection. If none of the lights work, it's most likely the ground connection. When I got my trailer, I had to run new ground lines to the lights. The original ground line to the trailer frame didn't work.
On an analog meter, check the operation by shorting the two leads (connect them together). The needle should move to zero. When you connect the leads to the trailer connection, if the needle moves close to zero that's good. No movement of the needle is bad.
Several years ago I wrote an article titled " Troubleshooting Trailer Lights" that might assist you in getting your lights working. Google: "Troubleshooting Trailer Lights Alan Glos" and it should pop up. The article is aimed at us amateurs who are not experts in such things. Good luck.