There were a few different climate air ducting arrangements for the 1st gen. Some only produce heat out the floor and defroster modes (like my 1981), some produce heat out the floor, defrost., and center vent only, some produce heat everywhere including both side vents (like the Crawler Truck), and some have fancy dash-mounted heater vents that direct heat towards the wing windows for a clear view of your side mirror. ....maybe parts of your system behind the dash are plugged, or loose, and half your air is just blowing out randomly cooking the components behind your dash

Because my blower fan only operates through my floor, center, and defrost. vents, when I crank that puppy on it's like being inside a F5 tornado. If I set it to the center vents only it's like I have an electric leaf blower motor in there. I believe a small electromagnetic pulse is set off simultaneously causing interference with near-by electrical devices.

Could it be possible that your fan maybe has some fried windings in its armature or maybe a half-fried commutator? I'll check with the Crawler Truck tonight to see if it also blows ya away like the motor in my 1981.

Also my heater works exceptionally well, nearly burns my toe nails off clean through my work boots.

Regards,
BigMike

Disclaimer: There is a slight possibility that I have exaggerated just a tad in this post.