Just bought an 8P A3 (non Bose), want to fit active sub

Hi there,
I'm new to the whole Audi scene, just bought a 8P A3 Sportback, non Bose. I have a 10" active sub from my previous car, and I wanted to hook it up. However, for the moment at least, I plan on keeping the Headunit and speakers original, and just adding the sub. What would be the best way to go about this?

Obviously I'd have to run power cables to the boot, and that shouldn't be an issue for me, but I was wondering where to tap into for the RCA output. Is it possible to bypass the factory sub, and get a signal from straight from the built in sub/amplifier/somewhere around there? Oh and the remote signal wire too.
I haven't opened up that area yet, don't really fancy messing something up a few days after I get it, so I'm not too sure what it looks like behind the carpeting. My idea was to disable just the factory sub, and redirect the audio signal that it gets to the active sub.

Any guidance would be greatly appreciated!

I like the sound of that adaptor kit! Can't seem to find specific details on the website though.
I had considered one of those stealth kits, but I did want a bigger sub, and plus I've already had this one lying around for a few months whilst I was in between cars. I had seen some custom 10" builds, but I reckon that's out of my depth. And I'd rather not start cutting bits off the car to make it fit properly.

If the adaptor kit will work with my active sub then I'd be very interested in it. Does anyone here have one/can provide some feedback about whether that will be possible?

Thanks!