Here's another good reason not to install tweeters in the B-pillars: The speed of sound.
340.29 m/s at sea level. So for the sound to travel the distance between the tweeter and your left ear at that speed would probably be somewhere around 3 milliseconds less than the sound travelling from the right side. May not seem like a large number, but it does have an effect and time correction wouldn't even be enough to fix the other problems that arise.
Your second problem would be the volume on the left side being much higher than on the right since the tweeter is little more than a foot from your head. I suppose you could adjust the balance to compensate for this, but heaven forbid you have a passenger. As you make it better on your side, the passenger side would get worse and worse. With my setup I have all the time correction and EQ settings set to my liking. When I have passengers, I just shut off the time correction. Because my speakers are closer to being equal distances from my head
Midrange -2.5 feet/4 feet, 1.3 milliseconds difference
Tweeters -2.7 feet/4.3 feet, 1.4 milliseconds difference
Although it's also not perfect, the time difference is much less than it could have been had I put the tweeters in my B-pillars right beside my head.
The ideal setup would be nearly impossible to achieve. You'd have to break out your windows and mount the speakers on poles. Nobody is going to do that, but the truth is, cars were not designed with stereo systems in mind. When working out your soundstage, you want the speakers to be as equidistant from your ears as you can get them. That is why kickpanels are such a popular option. They allow you the option of placing the tweeter and the midrange driver pretty much as far away as you can get them. I actually had my tweeters in the kickpanel location for a while
Although I initially thought the placement was good and it did sound good, people were kicking them when entering the car, that and I felt the sounstage was noticeably low. I have since moved the tweeters into the A-pillars, aimed slightly forward. I'm quite happy with this change, as the sounstage has moved both up, and forward.
