I found what appears to be an NOS trailer hitch wiring harness in a box of parts I bought in a large purchase from a Buick collector that passed away. I'm not certain what all it will fit, but it will definitely work on a 70-72 Skylark/GS/GSX, and other cars that use the same type plug for the rear tail light harness (see pics). The wires are not brittle and all the rubber plugs and dust cap is soft as this was inside a box with other parts for no telling how many years. There is not a box or package, but if you feel it has been used when you receive it I will refund your money except for shipping charges. $50 plus actual shipping.
There were two part numbers for the A body cars. One for the wagon and one for the non wagon. Gr. 7.068 and or 13.090 should get you closer. Matt