HAHA, I have the wheels from Larry Ward's car.
Mounting the tweeters in the vent is certainly the easiest place to mount because you don't have to drill a 1" hole, but what happens when you turn on the A/C and send air to the windows? You've blocked off both vents, so I'd assume more air goes to the windhsield vent.
I'm going back to Boston soon, so I'll be needing the heating system to function on the windows, seems like the corner vents don't really do anything, only the large one below the windshield has any affect.