After my engine is warm, the heater blows out cold air. What is the problem?

Check out this answer: http://repairpal.com/profiles/dan-o. This is a common problem in 2002 Highlanders as noted below. This happened to me too. This comment is from a technician on this site "I WORK FOR A TOYOTA DEALERSHIP AND THE PROBLEM IS IN THE TEMP CONTROL SWITCH, THEY DO GO BAD, THE SWITCH COST AROUND $600.00 YOU HAVE TO BUY THE WHOLE CONTROL PANEL. CANT BUY JUST THE SWITCH". I had mine tightened repeatedly and eventually had to pony up the bill for a total replacement for about $700.