to understand sound staging u have to understand how the human ear works. if the sound arrives at both ears at the same time, our brain perceives the source of sund to be in center (m not talking bout front or rear here). if the sound arrives at right ears first then our brain perceives the source to be on the right and so on.
the inherent problem in a car is that unless u have installed the system in a F1 car (highly unlikely :P) or in a Mclaren F1 which both have a central seating position, all the cars have the driver sitting either on the left or right side (depending on which country you are in). so the driver is always closer to one speaker than the other.
secondly most of the times the factory location for speakers and tweeters are not ideal.
lets for a moment assume that the car has no factory installation location for speakers. and consider a single point source of sound i.e. a coaxial speaker. ideally, if ur head unit does not have time alignment feature, u would want the speakers to be placed equidistant from bot ur ears. that is again not possible in most cars as the symmetry of the install will have to be compromised to achieve this. so u try to do the next best thing i.e. get the speakers to be at "about" same distance from each ear. that is why in most installs abroad u see the speakers being installed in the kick panels because the difference in the distances is minimized that way.
now add a tweeter in the mix and things start to get complicated. the whole idea of SQ setup is to have a setup in which speakers and components just disappear and u only have music. ideally, this should be achieved with a coaxial speaker as then our ear perceives it as a single point source. if the mid and tweeter are located within 1 foot of each other, our ear perceives it as a single point source. so if u install the mid in the factory position and tweeters on the dash, then again the staging will be effected. because different frequency sound waves travel at different speeds, u have to consider this as well.
if u have a sound processor, most of the new ones come with time alignment feature that will adjust the left and right channel so that u can have the sound from left and right speakers reaching ur ears at the same time. other wise, put the mid woofers in a kick panel or factory location and try to have the right tweeter on the dash as far away from ur right ear (in a right hand drive car) and then match its position on the left.
do remember that some tweeters sound good when they are pointing towards ur ears and some sound good when they are not pointing towards ur ears.
