2003 Tahoe Speaker wiring help

I am getting ready to add my amp and sub to the factory system (non bose).  I need to know where I can physically tap into the speaker wires to add my line level to rca converter.  OR  Is there a better way to do this of the factory system?  I know how/where to run MAIN power and ground wires.  Where is  the easiest place to connect the remote turn on wire (this can be anything that come on with the key)?

It's probably going to be easier to run a wire to the back of the radio than to remove the rear speakers to tap into those wires.  So I'd say just make all of your connections (with the exception of the main power/ground connections) behind the radio.  At the radio's harness you can tap into the 4 speaker wires for the rear speakers  (positive and negative for the left, and a positive and negative for the right) to get a signal for your line output converter.  The only drawback to doing it this way is that you'll need a longer RCA cable (probably 5m instead of a 1m or 3m) because with the converter located up behind the radio you'll still need to connect to your amp which I assume will be located in the rear of the vehicle.  Just make sure you run the RCA cable down the side of the vehicle opposite the main power cable coming from the battery.

Depending on what kind of GM radio is in the vehicle right now, you may need to tap into a switched wire in either the ignition harness or at the fusebox, because most of the newer GM radios do not have the +12V switched wire in the radio's wiring.

It just needs to be a wire that shows approx. 12 volts when the key is in the ACC or ON position, and no voltage when the key is in the OFF position.