That chassis does not use resistor . It is a transistor regulator circuit with a variable trigger voltage circuit from the Control unit
The test is to go to the 3 wire connector on the firewall , behind the booster . With a meter, test the yellow wire and ground. This should have from approx 2 thru 8 volts , a syou change panel settings

If yes , then the regulator at the blower is getting trigger voltage and is suspect.
The next test is to ground the blu wire at the blower and see if you then have high fan .. {This just takes the regulator out of circuit]
If yes, change regulator.
If No , you have possible binding fan bearings or bad brushes.