Defective heater resistor pack.
Blower fan resistor under glove box...by passenger's feet...!
Not that I have ever found.
The under hood fuse box is the power distribution center.
Sounds like a bad blower motor resistor block.
The heater blower motor is on the interior passenger side. It is kind of a round shaped housing facing the floor.
Check the blower motors ground and power in the truck and the blower motors resister......
Under the dash on the passenger side in front of the blower motor. You will see 5 wires going into a plug held in by 2 screws. That is the blower motor resistor.
most common problem is the blower resistor In the firewall on the passenger side of the truck is the blower motor speed resister coils, they burn out alot. I could not find one for my Dakota so I got one from a dodge caravan and I changed the wirring, that is the most likely problem
On the inside of the firewall, below and behind the glove box.
It is under the hood in the passenger side firewall.
30 amp fuse in fuse box.
Right side under the dash.