Stock tweeters have built in capacitors, not exactly a crossover, but more of a bass-blocker.

Your (any) head Unit is not going to be able to power components properly. It might work, but you run the risk of blowing your components. You will need an external amp that matches the rms of the speakers.

I disconnected my dash tweeters, for my coaxials, but others have said it really does not make a difference since they cant hear it anyways.

What year is your car. The windshield position is a bad place for tweeters and i would recommend you flush mount it in your door.