I know an installer who installs for the president and executives of Monster Cable and other high end audio companies in the Bay area, etc. He had a clever way of installing tweeters and sealed back midranges: He would use what looked like epoxy marine paste and stick them to the kick panel, positioning them for best staging, then letting the epoxy set, takes a couple hours. Then he would use that heavy wire mesh like is used in some contruction to form a "wall" covering the speakers, conforming the wall to allow for the hand brake, and securing it with screws. Then he used a fairly "accoustic" transparent cloth that looked like carpeting over the mesh. The finished product looked stock, and completely hid the speakers! Stealth! Really clean install!
Sealed midranges work out well because you don't have to worry about boxe sizes, leaks, etc. Dynaudio makes a fine quality one.
I am thinking of moving the radio to where the AC control now is, moving the AC control to the cigarette lighter area (after cutting the proper opening), then using the entire DIN area for a center channel for DVD-audio. That way I can use about the same speakers as for the sides, without having to use some crappy speaker, and so get some real good sound in surround sound.