All o the C6 schematics and plug pinouts are available HERE
If I were you I think I'd insist the installer use the add24. Print out the plug pinouts and the thread that shows how to add the RCA's to the add24. There are also plenty of threads that show the console removal, send him links if you don't get a warm fuzzy feeling from talking to him. When you get the car back, just be sure to check EVERY button and plug to make sure they all got plugged back in (heated seats, TC/AH, hazards...). If any don't work, make them fix it before you leave their lot.
FYI, the white wire at B3 of the main C1 plug is what should be used as the trigger for the new amp.