'05 Malibu heater blowing cold air

[sandsurfer]

Tried searching the forum to find an answer but wasn't successful.

Not sure when it started as I haven't used the heater in over 3 months. When either heater or Defogger is turned on, all I get is cold air.

The hoses going to the firewall are hot when car is warmed up. The resivoir is at the full line when engine is cold. I flushed the lines going to the firewall to make sure there was no clog. Upper and lower hoses going to radiator are hot as well.

Any ideas? Friends think it is a blend door issue but they haven't had a chance to look at it yet. Any other troubleshooting tips I should try?