The internal resistance of the blower motor often leads to other things in the system frying, but this isn’t the way to measure it. Instead, use an inductive ammeter to measure how many amps the blower motor pulls on “high.” The ’76-’90 CJs and Wranglers had only one wire going to the blower motor, so it is easy to figure out which one to test. And, since they ground through the heater box, sometimes a bad ground can increase the amperage needed to run it. To test for that, run a separate ground wire to the body of the blower motor. From ’91-’06, the blower motors had two wires; you want to test the positive wire (the not black wire) for most accurate results. If you see more than 20 amps, your motor is probably on its way out and it’s time to replace it.