It's different on every car. Somewhere on the door is most common, or you can use the little angle-mount cups that come with most tweeters on the dash. Here's what I am doing with mine:
Ideally you want to place them about as far away from you as the woofers. So if your door woofers are 3.5 feet away from your ears, your tweeters would be better off 3 feet away than 1 foot away.