Aye, I put a pair of subs into the doors, and powered them with an aftermarket amp under the passenger seat.
The LiveSound amp needed its own wiring loom, which was fiendishly complicated and could only really be used with a Toyota HU due to the connectors on the loom. I binned mine pretty swiftly after salvaging it from a scrap car.
It's an easy job to wire up an amp under the seat into the door speaker wires in the footwell connectors.
Rear tweeters are pretty horrendous, aiming high frequency hiss straight at your ears. However, the covers look pretty cool - much better than the fake louvre things.
All the wiring is there in every car for all these speakers. If you do get the rear tweets, make sure you get the mounting brackets and covers. The wires are taped just inside the holes under the louvre things. Same for the door speakers - wires will be in the doors already.
If you've not got the LiveSound door cards, make it your priority - they look better, and give you way more noise options. As I say, though, don't worry about rear tweeters too much.