Sure you can, it is really easy if Onstar uses like a 1/8th in jack. You can just get a splitter at Radioshack. Or you can just split the hot and ground wires yourself. Run one set to Onstar and another set to the AVIC.

Have you actualy done this yourself Retell ? the reason I ask is that these are Electret Condenser Mics and have a voltage up the centre conductor to power the Mic electronics, connecting the two units together without some kind of DC decoupling on one of them could damage one or both units.
Thanks Brian for your comment. I almost tried it. Has anyone figured out any way to tap in?
Have you figured out how to tap into the ONSTAR mic yet. I have an AVIC f700bt and a SAAB 2002 9-3 SE convertible and I can't seem to get it to work.
First, I tried finding the mic by following the wires out of the onstar button, but that got me now where.
Then, I went straight to the A Pillar in my Convertibe (which is where I think the mic is) and we --
got a radioshak 2.5 mm mic and chopped the head off. Then we connected the two wires coming out of our headless radioshack mic to the two wires that we found heading tot the onstar mic... and nothing happen. we even switched the wires... still nothing.
anyone have any input here?