When I turn on the AC, it initially blows out the dash vents but then immediately goes to the floor position. You can hear the door closing. I changed out the head unit with a spare I had, but it didn't help. There is a very thin plastic vacuum line that comes out the firewall between the heater core hoses that use to connect to the heater valve. It has broken off so close to the firewall I can't do anything with it. I had it capped off when it first broke and it worked fine. Where does that hose run to on the heater box??

It would be nice if you mentioned the year/model of the car you're working with.

Generally, all of the vacuum lines that operate the individual components originate at the push button control. If there is a break in the supply line to the control, the components will go to a default position and won't respond to control settings.

LOL I'm usually good with vehicle info. 89 Dodge Dakota 3.9 2WD

My old 89 Dak would do this on long grades as vacuum went away. As mentioned default position is floor and defrost for safety, as then the windshield wont fog.
I recall a vacuum pod for HVAC under the cab, unless I am dreaming. Been a long time since I was under that truck.

They have a check valve in the vacuum supply line to the push button switch. If that is bad everything responds to engine vacuum instead of holding a position.

**FIXED** Check out this video; https://www.youtube.com/watch?v=kHyKRDlx7JU