i have a 1999 Plymouth breeze, when i turn the heat on it don''t work, it blows, but it will only blow in what the temp from outside is. however, if i don''t turn it on, then after driving for a while, and i turn it on, it will blow in hot air for only a few seconds, then it starts blowing back in the temp from outside. what could be a possible problem and how much do you think it will cost too fix?

please note that i have already tried filling with anti freeze, coolant and replaced the thermostat.