Put a remotely operated relay between the OnStar radio and its power supply (plenty of room to hide the relay in the OnStar box ... and only 6 T10 screws to remove in most cases). When the time comes to make the theft, kill the radio. Enable it again later, if you see reason to do so. I'd bet a plugged nickle that the additional relay/receiver would never be noticed by folks processing the crime scene ... and even if they did, so what? It's not like relays and handmade radio receivers are traceable.