It is fairly easy to replace. You want the entire fan assembly (I think
you can just get the motor but I'm not sure if it is easy to replace just
that part). Cost for assembly was about $140 from www.volvoparts.com
(you will have to call them for the part, not listed on their site).

Here is a post where just the motor was replaced:

http://www.brickboard.com/AWD/?id=574873

Like I said, I replaced the whole assembly (which basically includes
the fan attached to the motor).

To replace it you remove the glove box, the trim under it and the
aluminum knee bolster behind the trim. I may have those listed out of
order (that is, I don't know if the trim comes off before you can remove
the glove box). You can then pull out the old one and bolt in a new one.
When the knee bolster goes back in make sure the bolts are tight or you
might introduce a dash squeek (I had to go back and tighten things up).
Took me a bit under an hour.