I have to disagree with "sttl013" on his suggestion for placement of tweeters.
I understand his reasoning for suggesting to mount the tweeter as close to the mid as possible, but this is not always practical or possible.
Although I am not an expert in the field, I have been following it for quite awhile & read quite a bit.
Tweeters, because they produce the higher frequencies, are very directional. In other words, if you mount them flush in the door down by the front speaker, then they will be aimed at your leg, and you get very little benefit from having installed them.
You want to have the tweeters aimed at the listeners head. It does come down to a matter of personal preference - if you mount them by the front mirrors, and aim them at the headrest of the closest seat, then the listener might find the sound too "bright"(too much treble). I personally am going to mount tweeters in the door(around the door pull) whenever I get around to adding to the stereo. (This is providing that there is enough space in the door to mount them in my preferred location)
The best solution would probably be to mount the mid & tweeter in a custom kick-panel fabrication, but this is a more expensive option. This way, the mis & tweeter would be close together, and both can be aimed at the listener.
That's my 2cents on the subject.