A common problem is that the heater resistor/ circuit board is defective and need to be replaced. The main symptom is that the heater fan with not run on the three low speeds but will run on high.
A blockage in the heater core is the most likely cause. This is a difficult and expensive repair.
you have a bad heater core
bypass valve not opening
There are a number of reasons why your cruise wont work on a 1999 Ford Pu. Your wiring could be fried.
Make sure the bulbs are still good.
Check your coolant. Is it thick and gunky?
my heater fan in front of my 2004 chevy venture van wont work
because it's broke
Possibly broke.
Could be: Low coolant in the radiator - or - thermostat stuck open - or - heater core plugged.
did your heater switch go out all of a sudden or did some work like high work and low not work? if some worked your blower resister is most likely bad under your glove box
The blend air door is probably broken.