If it is a good connection, then that is enough but it is always good to have extra earth cables in the system. I have ran a short cable from the socket to the rear bumper on my GQ. Since doing that, I've had a lot less problems with the trailer and vehicle lights. I also ran a couple of earths from the body to the chassis.
You need to check all of the wiring, and do away with any scotch locks. (those plastic connectors that you just place the cables in and squash them shut. They are prone to failure). Replace them with either soldered joints or properly crimped lugs or joiners.
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