I am not sure about the exact ground location but it is a common ground which I have heard is close to the License Plate area of the truck. Maybe someone else will come in while we are talking about it.
To me… here I go again, run new grounds if you can't be sure.
Test for grounding first through the sockets, see what kind of readings you get on a multimeter both in voltage and in resistance. A 12 volt reading will not always mean full current will get through, that is why I suggest a rewire when there is a problem.
Determine the power wires and then the ground wires and replace the grounds and don’t cut the ground wire off too short, you may need to do some clean up of the wire and having a longer lead helps.
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