Up high near the dash generally where you have room to hole saw and where you don't have any wires or window hardware behind. But, you should try to mount them higher somewhere to raise your front stage. What about your A-pillars?
as close to the midrange as possible. Aim it up towards your dome light. DO NOT put it on the A-pillar unless you have some form of time alignment as the imaging will be messed up as your highs will appear before your mids. If you can, get the braxial mount.
I try to keep them as close as possible, no more than 2-3 inches from the mid. I always mount them a tad above the mid and directed more to the listeners ears in a criss cross fashion.
The problem with mounting the tweeter up high in sound quality applications is the fact that the tweeter's timing is out of phase with the woofers, making the higher frequencies reach your ears first. You can't adjust the tweeters timing unless you bi-amp the system, which is difficult and requires perfect blending of power. As far as A-pillars, while good for added high frequency clarity, frequencies that require blending the tweeter and woofer (such as the typical female voice) will be off because the tweeter is heard faster than the woofer, making the soundstage less than perfect. Actually, competition winning vehicles usually use the component tweeter as close to the woofer as possible. This provides a centered soundstage with good blending of the drivers. Then, they may add additional tweeters up high and attenuate them just to raise the soundstage and give a bit more side to side definition. The reason "Professional" installations don't put them in places other than the A pillar is because most of them use factory door locations and use A pillars so they don't have to totally gash the interior to mount the tweeters, that and they don't often make kick panels. Mike started with the dash and didn't like it, so the A-pillars wouldn't be any improvement.