2001 Jeep Cherokee Heater problem
 

I bought a 2001 Jeep Cherokee Limited in July 2007 (93,000 miles) and just now needing to use the heater. Temperature guage shows normal temp. and blower works but heater won't put out hot (or warm air). I can turn the temperature control knob all the way up to highest temp but will only put out very little warm air, not enough to keep vehicle warm even on chilly mornings. What could be problem and how to fix? THANKS!