I wired up a sub/amp in my friend's 2009 b8 a4 2.0 with the audi concert. I tapped in to the power wire for the OEM amp, assuming it'd be a switched power lead. Apparently it isn't and ends up killing the battery. He disconnected the remote wire and his battery isn't dying anymore.

So what I need to do is either wire in a switch to a constant source and have him switch on/off his subwoofer every time he starts/stops the car, or
Find a true switched wire that will lose power when the car is turned off and tap in to that.

Any help?

There's a huge lack of wiring diagrams for b8 a4s out there.