Did you check the 30 AMP in line fuse that is connected to to alternator? That line gives full power to the blower when AC is turned on and on high blower speed.
Yes, the fuse is good. I always thought that if that fuse was bad the blower wouldn't work at all. Learn something new every day.
Anyway, I pulled it and it is good.
Sounds to me like your switch may not be making contact in the high position. The switch can be taken out, but it is probably easier to remove the heater controls as a unit. There are 3 or 4 screws that hold it in place, make sure you disconnect the cables and wires. Take the switch off the panel and carefull pry the tabs open and seperate the switch, pay attention to how everything goes together, there is a spring in there. Clean all the goop off the contact points and reassemble using dielectric grease. Easy peasy, the hardest part is getting the knobs off without breaking them.
I've had the dash apart several times. The first being when I got the car back in 97 to fix a couple of small annoying things, and then again when I took it apart in 99 to do the restoration on it.
I'm sort of leaning towards a connection at the switch too. Truthfully, I hardly use the blower since the car usually only goes out in nice weather with the windows down. So it's very possible that the connection at the switch could be a bit gummed up.
If that's the case it will be taken care of as part of a winter project.
Just switched over to the repro ram air manifolds and a 2 1/2" mandrel bent exhaust. The car sounds awesome going down the street. But even though I chose the quietest of the 3 muffler choices RARE gave me, it still has a slight drone at highway speed, so I'm considering Dyna-Mat (or something like it) as a winter project.
Since I'll be taking the seats, 8 track, and the console out if I do it, I may as well keep going and work my way up to the switch.