Oh dear, that sounds familiar to me.....

As @jriggs already sayd, itīs probably the Heater Blower Control (#3 in the next link) or the Blower Unit itself (#1).

Electric parts automatic air condition

prices are in Euro, donīt know how much these things are in $!

Iīve already changed some of these, u have to rip out the upper part of the dash board, no kidding!
I recommend to replace both of them.....if you just replace one part, it could happen that just a few weeks later the other one will brake down.....and then u have to do the work all again!