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Install a seperate "kill" switch for the heater blower.
-remove the center console side panel, drivers side
-find the brown wire going to the fan speed control switch
-cut it, add a "loop" line to a new on/off switch, mount under dash, in dash, on console, where ever you want.
-you now would have a true "off" position that GM forgot.
The later C3's don't have an "off" setting that works, the blower runs all the time.
As to your main issue of heat in the cabin, I just rebuilt the heater box in my 1972 this weekend, and can give you some suggestions:
1. Stop coolant from entering heater core in warm months. Install a ball valve in the feed line to the core would be an easy, Lowes fix.
2. Make sure the flapper door in the heater box is completely closed when in the "cold" position. There is an adjustable cable from your temperature control that moves this door. The door itself is somewhat insulated and the edges are rubber, creating a fairly effective block to the heater core when closed properly.