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I placed a relay (w/onboard fuse) by the fan and ran 10 gage from the battery connection on the starter motor--which is almost directly below the fan motor. The relay is activated by the existing high speed hot lead to the fan. I ran the ground wire to the starter motor mount too. What you end up with is about twice as much air flow (w/new fan motor installed).
The fan/motor runs through the normal wiring until the existing fan relay calls for high speed, then it switches over to the new relay where current is supplied direct from the charging system.
Once you put the new fan/motor in, close all the vents in the car, place a/c on with high speed fan, then feel for cold air spilling out into the engine compartment from the evaporator housing--you'll find a lot. Seal these leaks with black silicone caulk. The new fan/motor will create a higher pressure in your a/c ductwork--this will exhaust into the engine compartment unless the leaks are sealed.
I've done this and many other mods and I swear my '77 has the coldest a/c of any car I know--heck, on a sunny summer day I have to run some heat just keep from being uncomfortably cold--girls squeal when they plop their swimsuit covered bottoms on the ice cold leather of the passenger seat.