What a terrible installation job...
I'd personally go back to the dealer and politely ask them to remove it. There's very little chance a thief wouldn't immediately look for, find & remove that. OnStar has a better chance of helping you, as low as that is. Really seems like the only person that is benefiting is the dealership, who probably maintains some kind of administrative rights to monitor/track your car.
What dealership did you buy from ?