2003 Silverado HD stereo wiring ?

Just want to hook up amp and sub to company vehicle without too much intervention.  Truck came with basic CD player (no BOSE).

The best I can do is this pic of the harness that plugs into the radio's plug.  The wires you'll want to tap into are the two purple (rear right speaker) wires and the two green (rear left speaker) wires in this picture.  However, since this radio doesn't use a standard switched +12v wire to turn itself on, you'll have to make the connection for the amp's remote turn on wire up at the ignition switch harness.  A multimeter is necessary to verify that you have the correct wires so you should really consider doing that.

Also since that pic is not all that clear as to which wires exactly are the ones you have to tap in to you'll need to first eliminate the power and ground wires and then use a speaker popper on the 8 speaker wires to determine which wires are the correct speaker wires.  Finally, to get the correct polarity you'll want to test for voltage across each pair of speaker wires with the stereo turned on.  If u get positive voltage, the red test lead is touching the positive speaker wire.  If you get negative voltage, the black test lead is touching the positive speaker wire.