So I'm doing my first sub install currently, I DIY often but not car audio until I upgraded all the factory speakers last week.
Mini is a r56 2012 hardtop, base speaker package. I was planning on a full amp and 10 inch sub enclosure but I decided to try out this:

http://www.jbl.com/award-winners/BASSPRO+SL.html

Got a good deal on it and I can return if it sounds like crap, figured it might be all I need and I save lots of space if I'm happy.
I've already done the channel swap, other that that still stock wiring, just now I ran power to the battery through the grommet and grounded a cable in the boot, currently the JBL is plugged in, getting power...

Here's what I'm stuck on, without buying some overpriced dongle, I'm stuck on what to hook up the input with. Should I just splice into rear speakers for signal, or can I run the RCAs from the JBL to the X9331? (If so which pins because the only diagram I had consists of the rear/front left rights and ground cables.). I've read there isn't an RCA output in the stock headunit if I open that up so haven't been back there just yet. Or should I ignore the RCA and splice the high level inputs to something? I don't want to replace the headunit, and don't mind splicing but going for a factory look once its all put back together.

If anyones been in this spot and has an answer it would save me a hell of a headache tomorrow. Thanks.

 

You can simply splice into the rear speakers but it may be easier to pick up the rear speaker cables at the X9331.