2000 Denali Bose wiring specifications

I have a 2000 Denali and I am installing an amp and subwoofers, but I am keeping everything else factory.  I am going to hook up a line-out converter to get the signal to the amp. 

I need to know if I can wire the line-out converter directly from the back of the deck, or if I have to do it after the factory amplifier.  I do not even know where the amplifier is, or what wires would be what on it.  I also need to know what channels are already crossover so that I don't try to get bass from a high-pass signal.  A wiring diagram would be ideal, with locations of the amp and subwoofer.

I had originally wired the line-out converter to the rear speakers right at the speaker, but I found out that those speakers have a high-pass crossover already hooked up, so I got no base signal to the amp. 

Any information about the wiring would be greatly appreciated.