I worked in the parts room at a Chevy dealership back in the 70s. Then, they had a blower relay, that when not working it would do exactly as you describe. IIRC, when the high speed is selected it gets straight power. When any other speed is selected, it goes though the relay and is regulated down to the desired speed. This may not be relevant in your case, but I thought I would throw it out there.
Voted most likely to end up working at a car wash.
2009 Fleetwood Terra 34B - Ford V-10 running gear.
2007 AWD Ford Edge Toad