Where is the resistor located that controls the variable speed fan switch located on the on the dash for air/heat, I can only get high speed fan in either.
As far as I recall, this varies a bit from one make to another, but it is typically found on the blower motor.
It would help if you told us the Car and year.
On my GMC pickup the resistor was on the switch.
On many Chryco cars it is under the hood attached to the bulkhead, under the wiper motor.
It depends entirely on the vehicle. On my 1989 Toyota pickup it was a set of open resistive coils mounted on the ductwork next to the blower motor with the coils protruding into the ducting for cooling.
A Haynes manual, available at the local parts store, should show you.
That is on a 2000 Dodge Caravan, I was told it is in the fuse box or the power distribution box but there is no ID in either box for that dash switch.
Check behind the glove box. My '01 Town and Country has it mounted there. Empty glove box, fold out and down to check. They are easy to change, but a poor design. They default to high when resistor is overheated. Easy to do with temp. on high but blower on low. Check www.allpar.com for more info including advice on rebuilding with parts from Radio Shack assuming yours is same design as mine.