The trouble with a standard car is that you (the driver) sit to one side rather than in the centre of the vehicle.
This destroys the effect of 'stereo', that is, that two speakers ( left and right channel ) placed eqidistant from your left and right ears, produce sound that arrives at your ears at exactly the same time.
The further away from your ears you can mount both your speakers, the more accurate this effect can become.
As JDookie says, positioning the tweeters up high, e.g. on the dash tight under the windshield will give you a better 'sound-stage' as higher frequencies have more impact on the ears/brains perception of where the sound is coming from.
Often though, it isn't possible to get good positioning there because of the relatively short distance from your ears to the dash. Positioning your tweeters, way down in the footweels, on the kick panels say, pointing up toward the rear-view mirror in the top-centre of the windscreen, will give an improved sound-stage, but perhaps to some detrement of the 'stereo' effect.
To counter-act this, it may be possible to take measurements to a point just in front of your driver's headrest and fit your speakers such that you driver-side tweeter is close to half the distance away from your head than the passenger-side speaker.
Switching the phase (connect +wire to minus terminal and minus wire to +terminal on your speaker) on your driver's side speaker should then create a redistributed stereo sound, perfect for the driver and almost perfect for the passenger (assuming the measurements work out about the same for him/her)
Note: 'Sound-stage' is your perception of where the music is coming from. Out on the Hood, or higher up is good, inside the dash or console, is bad.