Re: heater blowing cool air
could be the heater core stopped up or the blend door on the heat-box not working. If the temp gauge is normal (half way or so while heat is on) then that would rule out the thermostat. Try warming up the engine, heater on, temp on hot and raise the hood and feel of both heater hoses running through the firewall, both should be pretty hot and close to the same temp. If not then that would point to a stopped up heater core. If both are hot then I would look at the blend door.