Dynaudio makes the high end speakers for Volvo. They are well
respected by Audiophiles. Go to google and search on
"volvo dynaudio speakers" for some interesting stuff.

Also see Jim Rothe's excellent page on this topic:

http://www.jimrothe.com/volvo/volvo-dynaudio.html

Others have also recommended crutchfield.com as a way to look up
speaker sizes. (Although they don't seem to list the dash.)

I wouldn't bother with tweeters in the back -- they really need to
be in front of your ears since high frequency is very directional.