Here's pics of the AMP I have - produced 9/11 and sitting here for months. I bought it to retrofit it to my 2008 M3, and had no idea that the wiring was different for the enhanced premium sound between 2008 and 2012 models. So it became an expensive paperweight for me unfortunately..
I would disconnect your current amp if there is water damage to prevent it from back-feeding the car or something to that effect.
The part number is 65127845163
Part Number 65127845163
Part Name AMPLIFIER FOR INDIVI
Online Price $1,141.34
That's the cheapest I was able to find it online. It's really a top notch high quality amp. Personally I wouldn't go aftermarket but that's up to you.
Make me a reasonable offer and it's yours.