Question Subwoofer installation


Good evening dear colleagues!

Even though my car is equipped from the factory with the Audi Concert system (front-rear speakers + a small subwoofer), I want to complete the audio line with another subwoofer. This is an active one (300w rms if I'm not mistaken) I have it from the old car and it sounds very nice, otherwise I'll buy another one :)

Has anyone done this operation before? I would like some advice on how I can pull the cable from the battery to the back, is there a plug or something? Where can I hide the cables?

Any advice would be appreciated! If possible, pictures would be great.

Thank you!

P.S: The car is an A3 8L 5 doors