My 2000 Chevy Astro,s heater vent is stuck in the ac mode, I only get cold air thru the vents. What is wrong?

The system is run by vacuum.There is a vacuum hose that comes from the drivers side of the intake manifold and runs to the penger side of the engine compartment by the A/C dryer.The vacuum line is plastic and runs over-top the thermostat housing.The plastic line gets hard and brittle and breaks.It looses its vacuum source and is stuck in one mode.You probably haven't noticed till now,needing heat,instead of A/C.