Get some strapping- the same stuff used to hold a deck behind the dash, and make a "bridge" over the mids in the doors(behind the grilles obviously), and mount the tweets on the bridge. This will provide a coincident source, so it does'nt sound like you have speakers all over the car. If you don't like that, try the kick panels. You'll get amazing imaging, as any speakers there have equal path-lengths to your ears. Thats the best possible place to put mids in a car. Well worth the effort.