to hell with ATC

in an effort not to make this too long this is whats up
97 lx, Automatic Temp Control. only blows out of defrost and defrost/floor. a/c works, when put on defrost. heat does NOT work. i believe i may have a bad heater core, just not too excited about taking this dash out. please help before it starts gettin cold around here!

Does the temperature of air change when you go from cool to warm? If temperature changes, the problem is more than likely a vacuum leak somewhere that is not allowing the doors to operate properly. If it blows out the defrost and not the vents, look for vacuum leaks. The system defaults to defroster if it loses vacuum. If the temperature of air does not change, there is a blend door actuator behind the glove box to the left that may be the problem.

its a vacuum line.

where exactly do these vacuum lines run from/to?? if i put the temp on 90 it'll only get as warm as the air outside. put it on 60 (in def mode only, not max a/c) and it blows cold. i'm told the sensor going bad is common as well. getting clogged, but it cant be cleaned and only replaced. is that true?