The judge will almost certainly order the return of the car (or a cash restitution) as part of the sentencing. If you don't return the car or don't pay the restitution you are in contempt of court.

More so, "possession of stolen property" is also a crime. The car doesn't magically become unstolen just because the theft went to jail.

For not returning the stolen car and using a stolen item? That's not illegal in the US? (European here)

Are you not stealing it again at that point? I'd guess that you would be charged for a similar crime, just at a different date.