Am amp/factory radio in 04 sierra problem

Im currently doing a install on a 2004 Gmc sierra with factory bose set up, xm radio and onstar.. I want to retain my stock deck, and door speakers, and tie in an aftermarket amp and subwoofer.. The amp is a brand new Fosgate 301m mono sub amp and fosgate 12 inch dual 4 ohm coil sub wired in paralell for 2 ohm load..   I got a line out converter (PERIPHIAL CONVERTER) and tied it in on my rear door speakers at the Hinge pillars.. The problem is that im getting little to no sound from the amp or sub and when I do the bass is all the way up with the gain all the way up on the amp and converter, and volume all the way up.. Im no rookie to car audio, and im pretty sure I have everything hooked up right..   Its almost like the signal is very week.. Do i need to tie converter into rear speakers and clip the speakers so they draw no signal or perhaps its because the bose radio system has its own amp and is supplying power to rear speakers thus cuasing problems... the rear speakers still sound fine when the amp is tieds in as well... I dont wana have to pay some one to do this, I work in a body shop and know cars very well so please help me if ya can

the bose system crosses over the speakers so you will get little or no bass from getting your input from these speakers.  do you have a factory sub in the vehicle? if you do you have to get your input from those wires , if no factory sub you have to get the input before the factory amp

Problem solved... I went ahead and had the local shop get it right...   On this truck the rear speakers are filtered so that they dont get the power and frequency that the fronts get...   So in short for any 04 Sierra, silverado and may apply to yukon, and tahoe a aftermarket amp line out converter must be tied in on the front speakers... The rear channels dont have strong enough signal for amp to run a sub... It cost me $60 but it saved me alot of head ache... Ive done alot of installs, but have never tried tieing anything on factory set up, so it was a learning experience..   All in all it sounds pretty good too