Wasting your time. It's NOT at all plug and play but more importantly they are a HUGE waste of time. Horrible sound quality once volume gets turned up a bit and you won't hear it in front seats.
Buy a used amp and sub in sub box.

I am currently modifying a factory.sub box to accept a shallow mount 10" MB Quart sub, but even that is a serious undertaking.
I should have just bought one of those carpeted sub boxes that were sold from Europe (like a JL Stealth box) years ago since they were cheap and available back then. None to be found on eBay anymore

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I swear, my cars are like a girlfriend.
Sometimes its a rough ride, sometimes its smooth motorin'.
Sometimes she doesnt like how i treat her and sometimes i dont like how she behaves.
BUT at the end of the day, she loves it when I am inside her.

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'91 850i
'05 X5 4.4i
'09 Clubman S