First thing Id experiment with would be somewhere higher on the door rather than on a sail panel or windshield pillar, closer to the mids than those higher locations will allow but not so low that the highs are coming from around your ankles. I am not a huge fan of kick panels, but then I drive a half ton pickup with a very upright seating position. I might be inclined to try mids and tweeters in kicks if I were in a vehicle with a more low slung seating position. Depending on what you drive kicks may work better for you. My tweaters are currently in the sail panels of my 09 Tundra and I will be experimenting with positions right around the door handle to see how they sound closer to the woofer.
For more coherent imaging the closer the tweeter is to the woofer, the better. The further away it is from the woofer, the more you may have to tweak the EQ because the higher frequencies from the tweeter will be more likely to reach your ears before mid range sounds. Ive never been good with jargon like beaming and phasing but to put it simply, spacing the two too far apart can cause certain sounds that transition from woofer frequencies to tweeter frequencies to become less stable in your soundstage. I hear it often in female vocals, where as the singer raises the pitch of her voice her location will shift on the soundstage as it rises and the shift from the mids to the tweeters becomes audible. In my opinion if you want to have some critical listening sessions female vocals with a good frequency range would be a big help. Id stop when you find a setup where her voice stays in the same place as it changes pitch.
Just general things to think about, but you really have to look at where your woofer is, what spaces you have to work with, and the acoustics of your vehicle.