Ideally tweeters should be mounted as close to the midrange speaker as possible. Some people prefer the in-your-face sound that come from mounting tweeters high in the doors, on the dash or in the pillars. But, most of those people don't realize it diminishes over all performance in some fundamental ways as well. Pointsourcing is a hot topic these days and there's alot of info to be read if you are interested enough to google (feeling lazy tonight).
Dash mounted firing at the windshield will give a relatively erratic frequency response. Reflecting high frequency notes off a rigid surface will do this, as it will create points where sound waves cancel each other out, and other places where they artifically reinforce themselves (depends on frequency, distance from the speaker to the reflective surface, material played and of course listener knowledge/experience.
Mounting the tweeters high on the pillars creates a situation where the midrange and tweeter are physically so far apart from each other, that our ears pick up this difference (our ears/brain can tell the direction sounds come from). When you consider that many times a note in a song will span a frequency range that requires both the mid and tweeter to partially reproduce it, having the two sound sources so far apart, sound sources that are trying to work together, will lead to problems with how it 'sounds'. Stereo imaging (that 3 dimensional image of the sound a pair of speakers in stereo can reproduce) will go right out the window. But then, many people are not that much of a critical listener, so they just think 'its louder' or 'more in my face' so that makes it 'better'. Im not suggesting those people are wrong, if you like the way it sounds more power to ya. But there are consequences to these mounting options, and they should be understood if an informed decision is to be made.
Its commonly agreed upon that the 'best' mounting position for tweeters (along with the mids) is the kickpanels. But, I realize not everyone wants to sacrifice foot room, build custom kicks or have speakers exposed to poor conditions (like the floor area in a regularly muddy 4x4) just for the knowledge that they have a stereo that images well. If that were my situation, personally Id find some tweeters with good off-axis response and mount them in the doors down by the mids. But Im more critical of car stereo performance than the average guy, even than the average car audio enthusiast.
Take into consideration what Ive said here, think about what's important to you (foot room versus better stereo performance, etc), then make a decision based on what YOU want out of the system. Anything else will just be guessing, or taking advice from someone else who may have a totally different set of priorities or personal taste in how the system sounds.
