believe it is under the dash on the passengers side.
the impeller are the fan blades themselves.
If you remove the breather box from in front of the blower motor you can reach the three retainers.
That would be caused by the blower resistor.
Blown fuse? Bad switch?
It is possible the blower motor fan is starting to seize up or is loose and catching on inside fan housing.
blower fan not working
Check the fan fuse. If the fan does not work on any speed then the most likely cause is either a blown fuse or a bad blower motor.
A broken blower fan. Had that on my Ford Explorer. Not a big deal to have a mechanic fix it, under $200
Could be either or, but also a switch, not to exclude wiring or connections.
The fan switch is probably bad. My Ford Explorer did that, I had to have the switch replaced. This could also be caused by a bad blower motor resistor.
From the www.2carpros.com website: http://www.2carpros.com/forum/1998-ford-ranger-blower-motor-vt87146.html "(The blower fan resister is) located in the engine compartment inboard of the blower motor and slightly below it."
under air cleaner assembly