It's best to have the tweeters close to the woofers but also as far away from you as possible... In a stereo setup, you ideally want the left and right speakers equal distance from you but as you can't sit in the middle of the car you need to put the speakers as far away as possible so the distance to each speaker is more similar.

Eg. If mounted on the sail panel, the left one is about 1.5m away and the right one is about 0.5m away from your ears.

If mounted in the kick panels, the left one is about 2m away and the right is getting close to 1.5m which is only 0.5m difference compared to the 1m difference in the above example. Even if mounted low, if the tweeters are aimed well they will appear to be high.

Nicco's mounting location will give good stage height and much better left right balance compared to the top of a door or sail panel but can sound very "bright" depending on the brand/model of tweeters. They also say "Steal me!" to a would-be thief :p

[Disclaimer - distances are estimated, not measured]

I had 6.5" KEF splits in the doors with the woofers in the factory location (with slight modification to the opening) and the tweeters in the sail panel with them facing slightly forward and up and found with those speakers that it worked very well with excellent height and seperation. Now I have Dyn's and they require different aiming so your best bet is to try different positions until you find one that works with your driving postion and your speakers....