As a veteran of several C4 Corvette audio builds, mounting the tweeters on the dash is the only way to good sound in these cars. You can buy pods, make them out of PVC pipe caps, or fiberglass them to the pillars. Sub's in the rear speaker compartments sound the best, but I have a friend who mounted a 10" sub in the rear compartment and really liked the way it sounded.

Good Luck, Randy