I've got my tweeters mounted on the a-pillars. Unfortunately, there is only about 10mm-ish gap between the plastic and the metal, so you can't really flush mount them. I just made a little metal bracket which I painted black and attached to the pillar and the tweeter, holding the tweeter at the desired angle...
I used to mount the crossovers right up under the dash, using cable ties. Now, I've got some new speakers, which have inline crossovers, so it's not a problem anymore.
Here's a pic of a tweeter: