hey everyone, i have a 1991 ford f-150 and when i am running the a/c the vents will switch back and forth to window vents to the floor to the actual vents and i don't know what is causing it?...also they are on separately not altogether

 

There's a vacuum leak or bad check valve in the system. Start at the engine and follow the vcuum lines to a check valve, reservoir and lines as they go toward the firewall. They usually leak somewhere under the hood.