my 2003 pontiac montana is only blowing cold air for the heat

see if it gets colder when you move to a colder setting, if not you may have a controller or blend door problem, if it does get colder I would check both heater hoses <going into firewall> and see if one is hot and one is cold, if this is the case your heater core is clogged and will need to be flushed or replaced