Jeep says to remove the negative battery terminal
Mine did the opposite it worked on the lowest speed only! I took the dash apart, by first
pulling the windshield vent plastic up towards the rear view mirror. Then remove the 2
screws on top and the one behind the ashtray. Now you can remove the center cover that covers the radio and heater sw.
Now removethe heater cluster and check the switch with a Volt ohn meter. the big green one is the feed and as you turn the sw you should be able to check the switch
moves to a different wire each time. My sw was NG so I replaced it! But I still was missing 3rd. So I checked the heater/resistor. Under the glove box is a plastic plate.
You will need a 5/16 socket or spin tight etc. Take the plate off and right behind the floor light is the resistor. It has 2-5/16 screws on it. I tested mine with the VOM cleaned it up and now I have all 4 speeds. I did clean the contacts with plumbers roll.