Tri,
Tweeter placement is a very subjective issue, and all down to personal taste. It also depends upon how much work you are prepared to take on.
The first thing I would do is to get a nice big bit of blu-tac, or something else sticky, and make sure that you have plenty of wire on the tweets. Decide upon a position, and stick them on. Listen to them for at least a couple of days, using every type of music that you're likely to listen to. Then move them, and try again. Keep doing this, until you've tried all possible positions.
Often, metal dome tweeters sound harsh, especially when on axis (firing directly at your ears), to remedy this, try pointing them more across the cabin, or at each other. Soft dome tweeters are more forgiving, and sometimes sound better firing more directly at you.
I tried mounting some tweeters in my mk2, using the dash speaker locations, but found that this also sounded a little harsh, not to mention the fact that you get a reflection off of the screen, which is not ideal. The poster who commented about keeping the tweeters close to the mids is correct, but this can lead to a low sound stage - again personal preference. The level of the tweeter and the mid should be matched by the manufacturer, and taken care of by the crossover - but the option is usually there to lower the level if required.
The mirror backings are the most obvious place to mount, and probably one of the easier options. I found that the best sound was produced with the tweets at the base of the a pillar, pointing towards the headrest on the opposite side.
