The point of tweeters is to have them equidistant from your ears, as your ears really notice the difference with the higher frequencies due to the shortness of the waves, so high up on the door is a bad idea.
Also keep them away from glass as this makes the sound too bright. On the piece of plastic just to the right of your feet is almost perfect, aiming to the centre of the roof between the driver and passengers heads. Don't aim the drivers tweeter at the drivers ear. Rather aim it at the passengers ear, and vice versa.
Hope that's understandable and helpful...
Here are some pics from Jamies car with the spot I mentioned.