Subwoofer Install

[TheTallMan]

Hello- first post. I just purchased a 2004 4.4i that I am very happy with. It has NAV and premium sound. I want to upgrade the audio with a subwoofer.

I spent part of the weekend reading things on this site- there is a very good article from alpac that helps with my plans. Also started to look around in the vehicle rear to see where everything is. I just have a few more or less basic questions, as a novice installer- please bear with:

- Instructions I have seen say that you should tap into the 15 pin connector wires (for DSP systems). I am wondering what exactly I am tapping into (what are those wires)? Are those the balanced output lines from the head unit, or outputs to a speaker, or what? I assume they are speaker outputs since you can take them directly to speaker inputs on an amp, but won't the level be too high, because they'd already be coming from the BMW amp? The wires on that connector look pretty thick compared to normal speaker wires.

- Can I use any aftermarket amp that has "speaker level inputs", or must it be a JL Audio or Zapco that is claiming to support balanced inputs? I guess I can also use a line converter (LOC), any ones recommended? Is that the better approach, going to RCA via LOC?

- I've seen comments that say there is a good place to tap power, in a green box down where the spare is. I didn't see that, only a blue box- I assume that is it, I opened it and it has taps labeled 75, 100, 50, etc. Lots of heavy guage red wire in and out...one of the taps labeled 50 is open, so I guess I can use that + a fuse in-line matched to my amps rating (probably 30-40 amps)?

- Is there a line from the head unit for turning the amp on only when the head unit is on? Or must I use a random switch 12v line (I think I found one in the compartment where the DVD player is)?

- in front of where the battery sits, there is a silver box somewhat off to the left. Many wires to it. Is this related to the audio, an amp of some kind? Or is the black unit in the rear left compartment both the "DSP" and the "Amp"?

I already have an amp and sub from my old car, just need to wire it.

Also wondering where I can get those neat wire taps. I found them on line, but wanted to just go to a store to get them.

Thanks!
TTM