Go for some small tweeters (no bigger than 1") that can be flush-mounted into the sail panels. Then, grab some 2-way passive crossovers (or make your own using some electrical bits and this link ) to split the signal going to your front speakers. The highs will go to the tweets and mids/lows to the woofers.

BTW, those tweets are huge.

1999 Ford Contour SVT #900/2760 (SOLD) 2005 Mazda 6s 2003.5 Mazdaspeed Protege 1998 Ford Ranger 4x4