What could cause the heater on your 1999 Ford F150 not to heat?

Low coolant, weak thermostat, plugged heater core/ and or( if it has) defective heater control switch.If system is low on coolant, fill to top and try it; but keep an eye out for a coolant leak. Take you hand and place it on the two heater hoses going through the firewall. They should both be hot to the touch when the engine is hot.If only one is hot, then you could have a plugged heater core. If engine temp gauge shows that the engine does not warm up, replace the thermostat. I hope this helps you.

Answer The most common heater failure on the F150 is operation of the blend door. Check www.heatertreater.net for diagnostic information and an inexpensive fix.