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BMW scrapped the rear tweeters in their HiFi/business sound a few years ago and I think the OP is only talking about the front tweeters. Indeed, on a standard setup, rear tweeters would not improve the sound at all.
As has been said, the tweeters plug into the mids in the front doors and you will need a new moulding (big one on the E90) and just the triangular piece for an E92. In either case, don't forget the foam filler piece as well.
Up till 2011, no coding was required for the Alpine upgrade because the Alpine amp used high level inputs (i.e. from the speaker wires and therefore amplified output from the head unit. For other amplifiers which take a low level input (unamplified) the car needs coding to tell the headunit that it is using low level outputs not speaker outputs.
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