i think (+1) means that the person agrees with you and says the same thing, in this case, he has the same question as you did, about the aftermarket Amp...

BMWs are wired wierd, which means that regular amps will not just go there, as the modules split the sound and the control of the modules - this way half of the electronics are in the front of the car, and the other half - in the trunk...

please search on this forum as there were successful aftermarket sound system installation, even though i personally prefer to keep that area OE - since the items are on the iBus of the car, the audio modules communicate with the rest of the car and the aftermarket units do not have that (unless specifically designed for BMWs)