2004 Chevy Tahoe HEATER FAN

MY HEATER FAN CONTINUES TO RUN AFTER WE TURN THE TRUCK OFF. IT HAS RAN THE BATTERY DOWN. MY HUSBAND WAS AT WORK AND DISCONNECTED IT SO THE BATTERY WOULD NOT RUN OUT. ALSO WHILE DRIVING IT, THE HEATER CONTROLS WILL NOT WORK PROPERLY; i.e. IT WILL GO FROM A/C TO HEAT.

WHAT IS THE PROBLEM?

F. Jones
Indianapolis