I have a 1998 f150 XLT extended cab with the 4.6L engine and my air (heat or A/C) only comes out the defroster no matter what setting I have it on. I think I may just need to get it blown out but does anyone else know of what it might be?
Thank you

 

Sounds like you have a cracked/broken vacuum line. It is a hard plastic line that goes by/behind the battery and into the firewall on the passenger side of the eng. comp. Even a crack in the line will cause this. Look close.