Not sure what year you have. I changed the one in my daughters 85 325E last year. Wasn't a bad job. Purchased a used blower from a local salvage yard for $30. You need to remove the cover (under the hood) to gain access. the blower is held by a metal strap with rubber bushings. Took me about and hour or so. Thank God you have a BMW and not an American car or one of those nasty hondas, where the blower is under the dash!
(P.S. Best tip EVER! if you don't own a Bentley manual, GET ONE!)