i just want to hear what soundQ has to say about mounting locations... im quite interested
Optimally, you should have the left and right speakers the same distance (also known as path length) away from the listeners. Since that isn't really possible in the car without major modifications, the goal is to minimize the difference in the path length between the listeners and the left and right speaker locations. This usually will result in kick panel locations as the best choice.
The relationship between the midrange and tweeter mounting locations should be as close as possible as well. In my car, I use point-source or coincident drivers for this reason, so that the tweeter and woofer generate sound from the same physical location.
Of course, time alignment and phase processors can compensate for bad install locations to a point, usually making the sound better for one location at the expense of all others.
One other point to make concerns the location of where to mount a tweeter in relation to the midrange (above/below/beside) if you have components. And this is going way back in my memory banks so I may not get it exactly right... but basically the focus of the sound from a component set will change depending on where the tweeter is located.
This is how it works... if you have a panel that has a mid and tweeter on it with the tweeter directly above the mid, draw an imaginary line from the center of the dome of the tweeter to the center of the woofer's dustcap. Now draw a line perpendicular to the first line and that is where the focus of the sound will be. With the tweeter above the woofer, the focus will be lower than the drivers because the perpendicular line will angle downward instead of straight at the listener. Have you ever noticed high end floor standing home speakers will have their tweeters stepped back a little from the front of the cabinet? This is why. They are trying to get the dome of the tweeter and the center of the mid on a vertical plane so the focus of the sound goes perfectly horizontal.
To apply this then to our cars, since the speker location is lower than our ears, you would want the tweeter to be below the mid so the focus of the sound goes up. Also, you would want the tweeter to be a little toward the rear of the car (for example, driver's side mount the tweeter at 7 o'clock relative to the mid whereas passenger side would be at 5 o'clock) so the focus of the sound is out in front of you. In our cars this isn't really practical since our 6x8 oval opening and grille tilt the other way. But even this can result in good sound (with the drivers side at 5-6 o'clock and the passenger side at 6-7'oclock). It is just that the music will be closer to you than optimal.
The whole a-pillar tweeter thing is another discussion entirely, one that I do not have much personal experience with. But, from what I have heard and observed, this location is best suited for very high frequencies only (above 8kHz) and used to give subtle sonic cues only and not as a primary "tweeter" location. Again, time alignment and phase processing can fix the bad location for one listener at the expense of others if a primary tweeter is mounted here.
Oh well, back to work I go.....