There's a couple of things here:
All 3 are well covered in the "Related Questions" below
usually on or near the top of the blower motor behind glove box area
In behind the glovebox.Easily accessible and an easy replacement if one fails.See "Related Questions" below for diagnosing blower problems, fuses, how-to replace and more...
behind or below the glove box
The blower fan / motor is mounted behind the glovebox. Remove the glovebox for clear access to the 3 or 4 mounting screws.
What you might be refering to is that the fan only works on high if this is the case your problem would be the blower motor resistor. this resitor is found on the blower motor casing it's about 2 inches long
Check to see if you are getting power and ground to the blower motor - if so blower motor is bad. If no power possibly fan switch or selector switch Also check the blower motor resistor, found in heater plenum.
If this question is referring to the blower motor resistor - fan operates only on HIGH speed - that's very well covered in the "Related Question" below. Hint: cheap and easy repair!If it's a different resistor, the question will have to be edited to include a bit more identifying information as to which resistor.
The heatercore itself is more centered under the dash.The blower motor is behind the glovebox.See "Related Questions" below for an excellent write-up on the 1986-1995 Taurus / Sable heater core
May be a loose connection or an open resistor? maybe a loose fuse in the fuse holder? Bad fan switch?
The heater/blower fan is directly behind the glovebox.Remove any covers that might be above the passenger footwell and the motor is right there - usually only 3 or 4 small screws hold it in.Btw, the resistor is in the same area also - it provides the ability to have multiple fan speeds. If you only get a high speed, it's very likely the resistor has blown.See "Related Questions" below for more
see a real mechanic
Directly behind the glovebox - empty the box, push in the tabs at the back to release it to a full down position to improve access to the connectors and mounting screws.