2006 Avalanche aftermarket amp w/ Bose?

I am installing an aftermarket AMP/Subs into a 2006 Avalanche Z71 with the Bose audio system.  The owner of the truck wants to keep his stock head unit, so I suggested the SLOCII Amp interface adapter from Stinger.  I've used it before in regular stereo systems without incident, however this time it is not working properly.  All I am recieving from the subs are mid to high outputs, no low's whatsoever.  I've even swapped out amps, and it is still the same.  I hooked up the subs to my amp in my car, and everything works as advertised, so I know it's not the subs.  Right now as per directions, I have the two inputs to the SLOCII coming from the two rear speakers.  I've tried swapping the negative/postive leads just incase i messed up the install, but it is still the same result.  The only other thing that I can think of is using the speaker leads going directly into the stock sub.  Is there anything else that you can think of, becuase as of now i can't think of anything else.  In addition, if I do have to use the stock sub wires, where is a good place to access them?  Any help would be great.  Thanks.

The speakers are sent a crossovered signal from the factory amp. You need to either grab the factory sub wires or you might also beable to put the speaker level convertor behind the deck and grab those wires as they should be full range.double-secret reverse-osmosis speaker-cone-induced high-level interference distortion, Its a killer