I have a 1991 Cherokee 4.0 with a heater blower that will only work on high. I replaced the switch and got the same results. Any suggestions?

After reading another post and doing some looking around I found a four prong connector next to the vacuum motor (at least I assume it was the vacuum motor) was loose and that fixed my problem. I had to remove a plastic cover underneath the dash on the passenger side to get to it.

You plugged the resistor that controls the speeds of the fan back in. Good job.