I have had that connector for a long time exposed to the elements, and no ill effects. They are also mounted on the front bumpers of many road service and tow trucks so they can jump cars easily without having to open the hoods of the service vehicle. The ground is between the trailer and the truck, when you run a 12V lead from the truck to the trailer it makes them a complete circuit between the two. If the connection for the ground between the truck and trailer isnt that good (rusty ball or trailer cup) then there is a lot of reistance in the circuit. When you raise the resistance of a circuit and the voltage remains the same, then the current on the circuit must go up to compensate and this builds heat into the wire. The plug I showed you may not be the EXACT one you need, just a representaion of what I used. I just did a real quick google search.