Were does the black wire go in the first pic?
It just seems to go around the outside of the diagram, but doesn't hook to anything.
Maybe you missed this (it was under the wiring diagrams):
Notice that in both systems the black wire (that heads down to the motors) is shown to be grounded - apparently to a 1/4" stud on the motor frame.
As confirmation that the black lead is intended to be grounded to the motor frame/mounting bracket, here is the wiring diagram for 1981 model year. The circuitry is still the same, but on this dwg it clearly states that the black 150 lead is to be grounded to the motor bracket:
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How do the black leads - in the harness that you have - terminate? Is there a ring terminal on the end?
BTW: It's probably just a typo, but the way you describe the motor up/down power leads is backwards:
I know dk blue and brown is pass side and blue white stripe and tan is driver side
All the schematics I have looked at have shown:
1. The LH (driver's) side motor being supplied by the dark blue/brown pair
2. The RH (passenger's) side being supplied by the tan/dark blue- white pair