Speaking from experience having owned a 2009 ForTwo Brabus Coupe with the upgraded speaker system including those tweeters and the underseat sub.... you're better off putting the tweeters on the A-pillar. I purchased a nice set of Hertz Audio component speakers for mine, and at first, had the shop put the tweeters in the factory location. HUGE mistake. The highs bouncing off the windshield just amplified them even more, which made the audio sound WAY too crisp. Took it back and asked if they could put them on the A-pillar, which they gladly did, and even they said it sounded much better.
If you're planning on trying to find the actual tweeters that came with the system, it shouldn't be too bad. If you're planning to buy some aftermarket ones, I'd say the factory location probably wouldn't be the best location.