1999 Taurus Heating Cooling

1999 Ford Taurus. Turn dial to internAL VENT, heat control all the way to red, air conditioner is on, no heat in car. Stuck door that swings from cool to heat? Is it vacuum controlled? Also coolant leaks thru overflow tank after car is parked. Have new tank on order. Suggestions? Comments? Anyone? Is the air conditioner supposed ot be on all the time?

After the car warms up, check the hoses going to the core and see if they are hot.