Thanks guys. I have had no problems with the fan on on highway speeds, its just one of my "nothing better to do" projects. I stopped by an auto accessory shop this morning and found the ideal solution. An inverted relay, ie it opens the circuit when it is energized. I'll wire the positive to the OD relay positive, so when the OD is on, the fan goes off. I normally turn off the OD when I get off the interstate, a habit I picked up when I had my 85 Mitsubishi Tredia (not available in US). It had two gear shift, the regular reverse and 1-4, and the second one to engage a mechanical OD (must use clutch or it grinds!). Yes, no lockout switch, so there's eight forward ratios. There's a mechanical lock-out for reverse.
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...and the bricks keep on rolling