I have a 2004 M-Van with a 4.3 V-6. The heat, defrost, AC control seems to be stuck. No matter which position is selected, air comes out of all outlets equally. I suspect it is either a vacuum problem or the switch is fried.

Most air blend door actuators are vacuum operated, so start by checking for vacuum leak, bad vacuum check valve or bad actuator.

The most common problem is a stuck or faulty blend door under the dash.

ck 4 a vaccum leak 1st key off dico the neg. batt. cabel hold it to the poss. 4 45sec. then re hook everything normall have the ac contols shut off

Look for vac hoses damaged or dislodged. System is designed to fail in defrost mode.

the blend air door is stuck look down the heat vents to see if something fell in there

Selection lever does not move to AC vent or floor vent only to the defrost. AC compressor does not kick in when AC is selected.
Vacuum level is at 18 instead of 23.
Any suggestions.

There is a vacuum leak under the hood. Find the vacuum line on the passenger side where it comes through the firewall and follow it to the engine. You system needs vacuum to operate and without it the default setting is on defrost for safety reasons.

this is caused by loss of vacuum from the engine to the ac control head.

heater box is warped doors not closing fully

FROM LUSTYKID:

You probably have a vacuum leak in the hoses connecting to Temp Control switch. Remove it carefully and inspect. Those things have mini -O rings sometimes. Any leak in any of the hoses and things go kaput. Check to make sure no leaks in the diaphram also.
As for the person with heat in the rear only, you need to check to see if the door is closing and opening. Not sure if yours is also motor control. If motor control replace the motor that operates the door. Is probably stuck in the hot position. But before you spend money ensure there isn't debris keeping the door from operating.