So you get airflow but no heat? Sounds like a few scenarios-
1. Possibly a mulfunctuning thermostat
2. Maybe the blend door is not working properly
see this link:
http://www.blenddoorfix.netfirms.com/
3. If you don't get enough airflow the blower motor is probably getting ready to take a dirt nap.
There is a resistor somewhere but I am not sure. That could also be part of the problem.
Check this stuff first
Good Luck