On most 70's trucks, there is a speed control resistor in the air box with about 4 wires attached to it. One of them goes to the blower motor. There should also be a high speed fan relay mounted to the firewall near the blower that bypasses the resistor assembly. The wire from the fuse box goes to the switch assembly in the heater control unit at the center of the dash. It's usually a 20-amp fuse marked "Heat" or sometimes A/C if the truck has factory-installed air conditioning.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!