I found this on another site & hope it helps.
Arthur in Texas
"If the vacuum system functions normally at idle but goes to defrost during acceleration, a small leak exists in the system. The leak can be located by turning off the engine and using a hand vacuum pump/gauge to check for vacuum delay while selectively blocking off vacuum hoses. I would start with the vacuum tank and check valve under the hood. This is the most likely location of a leak".
Loss of vacuum due to leak or bad vacuum caninster. Defrost is the default on this system and when vacuum drops it goes into default mode Check for leak either under hood or under dash area
It have battery
It is conveniently located on the passenger side forward surface of the defrost air distributor. To access it, remove the dashboard cover, remove the air conditioner controls and radio, remove all right side lower dashboard fasteners, noise deadening panels, etc, pull dashboard assembly away from firewall, rotate defrost air distributor slightly clockwise, and you can reach in and remove the module (grey) with a 7mm open end wrench this guy is exactly right.
Vacuum leak
buttons buttons buttons
The 2005 Chrysler 300 rear window defrost relay switch can be found beneath the drivers side dashboard. The rear window defrost relay switch will be near the center console.
If central air conditioner turns on but is not blowing through the vents, check the filter. If it is dirty, change it. Change the thermostat into cool, this will defrost the air conditioner.
You have a vacuum leak causing your vents to switch from dash to defrost.
You probably have a jammed or otherwise defective vent door.
The vacuum supply line from the engine is disconnected or broken. When the vacuum supply is cut off the system defaults to the defrost mode.
When you select the defrost setting , the air conditioning compressor will kick in and out to help dehumidify the air , that's normal to hear the compressor kicking in and out
When there is a problem in the vent system it defaults to defrost setting. Check the doors & vac supply