If it was me, I'd buy the kit and fit it a bit at a time. probably start with the window trims / mids and tweeters. Once that's done, running the wiring through the car is the next step.
The main thing to keep in mind with this system is that it's all plug and play- no wire cutting! BMW will even provide you the instructions. Hell, ask for a copy of them before you buy, to make sure your completely happy first.
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