18” car hauler, what are you hauling matchbox cars?

Sorry I just couldn’t resist.
I missed what you got for compression on your tests, if was real low it could be the problem as could a bad plug wire, or plug.
The wiring problem can be tough, this site can be of some help.
http://www.etrailer.com/faq/wiring.aspx
The easiest and quickest way would be to put in a new connector that just plugs into the original harness.
http://www.etrailer.com/c-VWH.htm