So you have Onstar but no Monsoon
Metra's GM radio replacement adapter lets you replace the factory radio in select OnStar-equipped 2001-up GM vehicles using the existing wiring and connectors. Even if you don't subscribe to OnStar, you'll need this adapter to connect an aftermarket stereo.
Hook the GMOS-01 to your aftermarket radio, mount the module, and plug it into the existing factory plug. Get the great sound you want, and keep all important safety features intact. And you can adjust volume settings for OnStar as well as the safety chimes. Also, once you turn off the ignition in these GM vehicles, the radio continues to play until you open the door. This interface maintains that function with your new aftermarket radio.
This was taken directly from Crutchfield, and I don't believe anybody makes an adapter that does it all, that being chimes, Onstar, and steering wheel controls so I would see if the dealer you purchased the GMOS-04 from will swap it for the GMOS-01, if not you could always sell the GMOS -04 on E-gay (for a loss of course) or something like that. GOOD LUCK to your and your endeavor.
