There is a black saturn astra that has been put up for sale on Craigslist in the Chicago area:
https://chicago.craigslist.org/chc/ctd/6185327208.html
I've saved a screenshot in case it gets taken down. If any of you want to check around Illinois or neighboring states for used car listings, that would be some good sleuthing and a way to help out. Don't contact the listing but refer it to law enforcement. I'm sorry I'm only doing this today, June 21st, almost two weeks later, and can't check every listing in the area alone, but there's a good chance some kidnapper would be trying to sell his car and get rid of it now that the make has been identified.
The place in the listing I referenced looks like an all-cash on-the-downlow type of used car place, in a poor urban area. Some operation like this or another 3rd party somewhere could wind up buying the vehicle and not even know what they have.