My thinking on this is that the perp stole the car, then stole plates from a different vehicle, which he put on the car.
In this scenario, it's quite possible that the car belongs to someone affiliated with UI who is from out of state or abroad, in which case it still wouldn't be that difficult for LE to identify the legitimate owner. The identification process would likely be made even easier if the car has been reported stolen to local LE, but students and faculty members often leave their vehicles behind while they do internships/ fellowships, go on research trips, etc. for the summer, too. It's also possible the car is registered with the university, an apartment complex, etc. for parking purposes. Same concept can be applied to stolen plates.
Even if LE has found the legitimate owner of the Saturn Astra, unless there is a clear image that shows the perp in possession of it -and if LE has found such footage, I'm inclined to believe they would have have released it by now-, it would be difficult to find the driver, IMO. It's also possible the perp already had a second set of stolen plates even before kidnapping Yingying for post-abduction purposes.
More info on the Saturn Astra:
"General Motors predicted sales of between 30,00040,000 vehicles a year in the United States. Sales were slow, due initially to a large glut of unsold Ions and later the uncertainty surrounding the Saturn brand's future. In 2008 only 11,968 were sold in the US and in 2009 this fell to 6,298.
...
(I)n 2010 General Motors discontinued the Saturn brand."
https://en.wikipedia.org/wiki/Opel_Astra#Saturn_Astra
(Redirected from Saturn Astra)
For comparison purposes, in 2008 a total of 351,007 units of Toyota Corolla -same class as Astra- were sold in the US, and in 2010 -the year it came out-, a total of 24,495 units of Chevrolet Cruze, also a compact class vehicle, were sold in the US.
https://en.wikipedia.org/wiki/Toyota_Corolla#Sales
https://en.wikipedia.org/wiki/Chevrolet_Cruze#Sales