The reason that the rear tweets are mounted in that spot, is beacause:
1. I sat in that spot, with the door panel off, moving the tweet around and it sounded perfect at that location.
2. Mounted to far forward, your knee blocks the sound and DENTS it: Not Good.
3. With the rear windows down, I dont loose my highs, per say if they were mounted higher in the rear piller.
4. Any rain (if any) falls while window happens to be open(driving), its protected by the arm rest.
Im just giving some ideas to other TB owners. Overall in the end, its really up to the owner and what they want.