How do I add an amp (for sub) to B9 S5 Cab B&O system?

[Jeff in MN]

I have an '18 S5 cabriolet with the B&O system. There are two factory subs in the trunk area (unlike the SB/coupes, which have one sub in the rear deck, I believe). The subs aren't accessible without taking out a significant portion of the trunk lining, and I really don't want to do anything that could potentially interfere with the motion of the roof after I put it all back together. I'm considering adding one of these downfiring Kicker subs (https://www.kicker.com/comprt-10-inch-down-firing-loaded-enclosure). I like the low profile and the reviews seem pretty favorable. I would power it with a Kicker Key500 amp (https://www.kicker.com/key-5001-1-channel-smart-amplifier). It seems that most people who add a sub tap into the wires at the factory sub, but I'm hoping I can do that at the factory amp instead. I can access the amp (I believe) in the side panel of the trunk, but would probably need to outsource the job if it has to be done at the sub end.

Can anyone help me identify the correct wires in the harness for this vehicle (and confirm that I'm heading down the right road in the first place)? I have other plans to swap out the door speakers with something more substantial, but my goal with this request is to add some lower-end punch to the B&O system that I just don't think the factory subs are providing. I would not be replacing them--I'm simply hoping to supplement them on the low end. I just need some guidance to understand how to get the signal to the amp.

Thanks,
Jeff

[Bobby Kinstle]

You'll need to cut the wires leading to the factory sub. I suppose you could cut the wire after it comes out of the harness on the amp in the trunk but it's a big bundle of wire and better hope you get the right one. I have a wiring diagram in my B9 upgrade guide.

https://docs.google.com/spreadsheets/d/1ftkRt8rz1ZgtTtAs-NRYAIkHbtRuhX2cu702CeuqhVs/edit?usp=sharing

BTW it looks like the Kicker Hideaway HS10 also can connect to the car's sub output just like the Key500.1 Might be a good alternative for you.