Use some double sided tape to position the tweeteres in a few places and have a good listen each time. Put them where it sounds best to you.
Putting tweeters away from the mid-range doesn't mean bad imaging. With a good DSP you can set the delays up accordingly. Someone on here uses Focal Utopia tweeters in the A-pillar itself right above the dash facing each other. He's won a competition of two.
Now, I put mine in the defrost vent covers for reasons a) raise the sound stage and b) cheap to replace those covers at $7ea if I ever changed my mine, or sold the car. I think the setup sounded pretty good, although the titanium dome tweeter was harsh with the windows up. The combination provided smooth response with great depth, more than any other stereo setup I have owned home or mobile. Hopefully that changes soon with my wife's truck which better drivers, just need to power them.
