trouble adding sub to 07 mazda cx7 w bose

Im trying to add an amp and sub to my 07 mazda cx7 turbo, its the one with the bose speaker package in it.  I wanted to add a sub to give it that finished filled out sound.  So I picked up a 12' Sony Xplod and an 800 Watt Sony Xplod amp to drop in my hatchback.  I went to 3 stereo shops and they all told me to add a hi/lo converter via the back speakers.  I bought the PAC SNI 15.  I was told to hook into the rear speakers with the hi/lo converter and I could run the rca to my sub from there.  So I pulled my rear 3" speakers out of the rear hatch area and connected the hi/lo converter as the instructions on the package said.  When I went to test the sub, the amp lit up but the sound at the sub was muffled and weak.  It also seems to have lowered the volume on my bose speakers, they sound weak now to.  When I pulled the sub and amp off the bose system sounded like it went back to normal.   Is there another way to connect the hi/lo converter?  Would I be better off trying to jack in at the rear of the factory head unit?  Can anyone help me with the factory wiring color scheme for my mazda?

I believe the problem lies in trying to tap into your rear 3.5" speakers.  The factory most likely uses a high-pass filter on such a small speaker.  Try accessing the wiring before the amplifier behind the factory deck.  This signal is most likely a clean, unfiltered signal that travels to the factory amplifier.  If there is nothing more than a data cable behind the CD player which runs to the BOSE amplifier, you can tap into the woofers which may be in the rear doors?  Metra's website tells me that your vehicle has two 9" subwoofers in it already.  If that's the case, you can also grab the signal at those speakers, which is already a low-passed signal.  You may also want to purchase a stronger LOC.  The SNI15 is only good up to 15 watts of input.  If you're tapping into the signal after the BOSE amplifier, there's a good chance that you're dealing with more than 15 watts of power.  Try an SNI35 OR an ALOC60, which I believe is good to 60 watts.  Hope this helps!  Good luck!

OK thanks for the info.    It will be helpful when I start digging into my truck again.  do you know where i can get wiring details for my truck so I can hit the right wires?.  .