Here's a couple of creative ideas I've used that don't require any drilling or modification of the door panels.
The first is to use double-stick tape and place the tweeters right above the power door lock/power window switch. You just remove the door, pop out the lock/window module, run the wire though, and put the module back in (the wire is thin enough that it won't affect how the module sits). Then you tape the tweeter so it sits right over where the wire feeds up so you can't see it. Sure the tweeters fire more vertically than horizontally, but a lot of car tweeters have a very good off-axis response (and are sometimes designed for that).
The second idea is the ultimate in stealth: use the aire vents! I ran my wires up the opening of the vents, removed the dash and vents, and made little metal brackets to hold the tweeters that screwed into the vent tubes. Then I put the vents back over the tubes and put the dash back on. This sounded great and kept a totally stock appearance. The downside is that the air flow is a little restricted depending on the size of your tweeters.