The best spot for tweeters (and components) is in a kickpanel. The sound goes right to your ears and is nice and crisp. If you can't afford that then I would just throw the tweeters in the A pillar. I did that in my winter Ranger with little cheap ass ebay tweeters and they sound good.

98 Lime Green SE w/SVT Kit, Projectors, CF Hood, 18" Rims, GC's, Shaved, System 98.5 Silver SVT #5737 Black FSVT Rims, MSDS, BAT Drop 85 Toyota PU Lifted 06 GSX R 600 Black/Silver My Rides