03 Z71 Tahoe with Bose: Help!

You don’t need to go to the amp to do all of this. The easiest way to do it and to be sure you have the correct wires is to tap into both rear speakers right before the actual speaker. Then connect them to the high to low converter and you will have your signal. Then run your power and you should be good to go.

It's not available yet, but hopefully soon.  PAC is making an adpater that should simplify the install:

The C2A-GM24 allows you to add a 2 channel aftermarket amplifier for GM vehicles with Class II data bus. What makes the C2A-GM24 unique is that is takes the fixed level audio signal from the factory head unit before it gets to the Bose® amplifier and gives you a variable output. The C2A-GM24 outputs are digitally controlled by the headunit's volume knob. You get full range audio signal, not a Bose® specific equalized output, highpassed or lowpassed output.

http://www.pac-audio.com/products/oem1.htm#c2agm24

I am having a similiar problem.  One - I have a 2003 tahow with the bose sound system.  I just bought a stock 2002 sub enclosure from ebay.  I would like to add an amp and a sub.  Is the C2A-GM24 what I need for this? 

2 - I would like to use the AUX function on my stereo. I know that this is usually used for the XM radio but there has to be a way to connect something else to this input.  Maybe an MP3 player.

Apparently the c2A-GM24 is what you need to do it without tapping into any wires.  I used an SMI-15 and it works great and was less expensive.  I caved in and bought a PAC Tr-4 to create a remote wire.  It was 13 bucks, and now I don't have to rig anything up through the fuse box.  The strereo sounds great.  I just have a Kicker Comp 10 in a sealed box.  I didn't feel like messing with that factory sub spot in the back because it will only accomodate an 8" sub.

The SMI-15 (from PAC) was extremely easy to wire into the Bose amp outputs.  Access was easy through the center console, and you tap into the wires on the harness with the blue clip.  Tan/Grey, and Light GREEN/ Dark Green.  The remote wire was the thing giving me the headache, but the TR-4 solved that easily.  Wish I could have gotten a c2A, but this worked just fine.