The ground wire is a short, black wire contained within the hinge. It's riveted to each half of the hinge, but usually fractures from the flexing.

On all my wagons, I removed the remains of the original ground wire and installed a longer ground. It runs from under the headliner (near the hinge), through the hinge alongside the harness, and to a convenient tailgate chassis screw near the wiper motor.

For wires in the tailgate harness, I use instrument test lead wire. It's highly stranded, so withstands a million flexes. And it's rubber insulated (not vinyl) so stays flexible in the cold.
--
Don Foster (near Cape Cod, MA)