I could be wrong, but I thought the 70's window motors were not wired like the later ones. The window motor gets it's ground through the case, then both of the wires are 12+. One for UP and one for DOWN. I don't believe that it's a switching ground system.
I know for sure that's how the K5 and Suburban tailgate window motors operate though, and they were used all through production, including the 70's. Iirc, they are similar in design and use the same plug as the 70's pickup power window motors too. Although I haven't messed with them in such a long time, and only a time or two. It's not a common option to find on the older trucks.