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I'm aware that ideally, tweeters should be placed high and aimed for best effect at ear. My Q is: How critical is this aspect?
I'm currently planning on installing 6.5" components in my C6's doors, and I'm leaning toward placing the tweeters inside the area where the factory 3.5" speakers are, mostly to avoid having to modify the appearance of the car's interior. Would this be acceptable to your ears? Why/why not?
Couple other things to consider, on axis as Kale mentioned vs off axis. On axis with the tweets aimed at your head can be painful depending upon the power and tweeter type that you have, if not down right brutal and overpowering. Most tweets are pretty favorable from a 30* off axis position and are often designed that way for car applications.
Best bet is some of the Blu-tac or similar material and a bunch of speaker wire, and mess around with locations while listening to material that you know VERY well.
The car is an unbalanced and unforgiving audio environment, you have to decide for yourself what level of effort YOU are willing to make for your particular hearing tastes and willingness to modify your interior.