Aftermarket Amplifier install question

[GrindrX5]

Hello all,

I'am finally a proud owner of my first BMW! I recently purchased a 2002 X5 4.6 and am loving every bit of it. I find my self looking for reasons to leave the house so I can drive around. I love it, it is a true driving machine!

I only have one issue and that's with the sound system. I have been looking through tons of forums as well as the rest of the internet and I'm having trouble finding answers to a couples of questions. I have looked through the how-to's maybe I'm over looking something or just not getting it.

...I plan on keeping stock head unit and would like to keep it in place while working from the back only. I want to upgrade stock amplifier and also add additional amplifier for a sub. My X5 previously had a Dice Electronics upgrade, not sure if this will make a difference.

I'd like to use existing wires if this is possible, my question is about the RCA's. Where are they and how to I get to them?

Is there some sort of harness, or way to splice into the signal. I'm not ok with using a fm transmitter, I'd prefer a superior signal.

If somebody could point me in the right direction your help would be greatly appreciated.

Feel free to ask any questions.

Thanks in advance!