If the brushes are shorted they will be pulling essentially stall current for the motor. This will trip the thermal protect circuit in the regulator (protects regulator if too much current is drawn or the regulator is asked to dissipate too much heat (look at the heatsink) The regulator will vary the current flow thru the motor to ground. That's why it is in the ground leg of the motor circuit. If the voltages are present on the firewall plug, then yes, it is possible that the regulator is shot. But, you are saying that the blower squeals or in other words, it's working but not very well. This sounds like a motor problem, not the regulator. The regulator is a heavy duty component and not too complicated, or either it's working or it's not.

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Jeff Lawrence
1989 300e
2000 Dodge Grand Caravan SE
No matter what you fix, there will always be something else to fix..
"Warranty" is just another way of postponing the inevitable.