Keep Onstar with aftermarket radio on 2009

[badtzbuzz]

I haven't been able to find a solid resolution to this, but does anyone know how I can replace my factory radio (without Monsoon) while keeping at least the Onstar audible prompts and microphone? I've searched and asked some audio installers, none seem to have a real answer.

I've checked the Axxess / Metra adapters, and the GMOS-LAN-02 works, only if the system has an amplifier, I'm assuming I don't, without the Monsoon. No other adapters work on the 2009.