Line out converter

[lo-lyf]

2004 ext cab silverado with bose and onstar. I have seen something by PAC that plugs into the factory stuff for iPod, but didn't know if there was a line out like that.

Cool, too bad it isn't an 07+ non-bose, I have a harness that could be cut and modified to do what I am bout to tell you how to do. I have never done this before so I cannot verify that it will work.
the things you need:
metra 70-2003 gm00-06 aftermarket radio harness (female)
metra 71-2003 gm00-06 factory radio harness (male)
(2) line out converters, three if you want separate sub control

Wire your 71-2003 and 70-2003 together leaving the speaker leads disconnected (I would verify both visually and via multimeter to make sure all wires are in their correct locations and all factory wires have a wire on the aftermarket harnesses). The line output converters will wire to the speaker leads of the 71-2003. The speaker leads on the 70-2003 will be for your leads coming from the amp out to all four speakers.
There is a harness directly behind your a/c controls attached to the back of the subdash. This harness is for bose. Disconnecting this harness and plugging in the 70-2003 will completely bypass the bose system. I believe (not 100%) that bose and non-bose use the same radio, I would verify before proceeding with this venture. Your onstar and auxiliary will be in the smaller harness of the two. I hope this helps and if I wasn't enough help, sorry in advance.