I just tapped into the pre-BOSE amp wiring to add a sub to my 05 G35x. These are the wires I tapped into:

Right Rear Speaker
+ BLUE
- PINK

Left Rear Speaker
+YELLOW
-BROWN

What I did is splice the ends of RCA wires into these wires. Just to make sure the connections were good before closing everything up, I ran an RCA from my Infiniti to my other vehicle, and plugged it into its mono sub amp. Here's what happened:

<18 volume subs play normally
19+ volume subs are hitting out of phase, doesn't sound like music, just random hits. Almost like bass static.

I was like WTF so I played some test tones on my iPod in the Infiniti. Up to volume 18, everything sounds normal. With each incremental increase in volume up to 18, the subs get proportionately louder. Then, when I hit volume 19, the subs get MUCH louder and sound like they are clipping or something.

It isn't the amp or the subs, because they play normally in my other car at any volume.

Any experience with this? I'm completely mystified.

 

Try reversing the polarity on one of the subs and see if that helps.