The most common cause of the heater blower being on high only is the blower resister failing. The second most common problem is the blower switch going bad.
My 1992 Saturn has heat until you change the blower speed what is the problem?
Probably the blower motor resistor is shot.
under the blower motor housing. to the right
The blower motor speed switch is burned out or the fuse is blown.
Maybe Hi-speed fan relay defective and not opening
specs on how to replace a heatercore on a 1993 ford explorer
The blower resistor is located behind the dashboard. It is found behind the heater near the center of the vehicle.
Looking at the picture in my Haynes repair manual it looks like the blower motor resistor is in the engine compartment mounted just to the right of your blower motor when looking from the front of the Explorer - Helpfull
usually mounted on or next to blower motor
If yourtalking about the heater blower, I believe they are hidden behind the glove box inner liner.
The blower motor is on the right side of the engine compartment on the firewall.
Most likely and bad blower motor resistor.