air circulation is stuck on defrost vents only,

you have a vacuum leak most likely under the hood next to the firewall on the passenger side,start the engine(listen for hissing noises..air escaping).turn the A/C on high speed,select A/C or Panel(front vents).start listening and feeling around on the smaller hard plastic hoses first,check ALL the vacuum hoses,you'll find it...repair the leaking,split,dry rotted line/hose.vacuum hose and fittings are cheap at a parts house! when you have a vacuum leak,the blower motor defaults to the defrost mode!