My fan only blows air through the defroster. What causes this? How do I fix it?

you have a vacuum leak,most likely under the hood next to the firewall on passenger side. probably the smaller hard lines,but check them all.should be able to hear it/them leaking air with engine running.repair fittings/hose at about any parts house,cheap.when you have a vacuum leak,the blower motor will default to the defrost mode!