Ok first, Does the blower motor slow down or does the air stop cooling? Vacum has nothing to do with the blower motor, it is 12 volt :!: :!: If the air flow stop's there is a plug on the drivers side of the heater box, it connect's the engine vacum to the box, it has a Red, Yellow,Green, Black vacum lines, Ford had a problem with these connectors leaking. The vacum line going thru the firewall to box should branch off of the brake booster, if the booster is leaking vacum, it will starve the box blend door,floor,defrost,panel, accurator's when you get on the fuel.... HOPE THIS HELP'S :!: :!:
''JUST MY TWO CENT'S WORTH''