A/C blower

Measure battery voltage with the engine idling and with the speed increased. If the voltage goes up with engine speed about all you can do is make sure the serpentine belt is tight. Normally if the spring-loaded tensioner rusts tight in the relaxed position you should hear some belt squealing too. Other than that, it's normal for the voltage to increase a little with engine speed.

You might also want to have a load test performed on the alternator. If you can only get about 35 amps out of it, which is roughly one third of its rated capacity, it has a shorted diode. Most commercial load testers also test for "ripple". That will be very high if one diode is defective.

Sounds like the alternator is not keeping up with the additional load coming from the blower motor load-

Sounds right.

The resistor you're probably referring to is for the different speeds. That's not going to affect the speed changes from engine speed changes. If the alternator is related to the problem you should see the head lights getting brighter too.

Just to be sure we're talking about the same thing, you don't mean the air switches to blowing out of the defroster vents instead of the floor, do you? That has a totally different cause and fix.

Sounds more like a problem with the mode doors. Behind the control panel is a computer that controls the actuators. That is needed to provide the 10 percent air flow there, 20 percent over there, and 70 percent somewhere else. That gimmick adds a lot of complexity and unreliability to what used to be a simple common sense system, but that's what sells cars today.

I doubt the motor is failing. When they do, they usually fail to start up. They rarely fail once they're running. The fan will sound louder if multiple mode doors close to block air flow. If it seems like air flow is reduced I would suspect the controller first. I really hate throwing parts at a problem but there's no easy way to diagnose the system other than trying a new or used controller.