Can you repossess the car if payments aren't made on a private loan?

It depends, if you did not have a security agreement on the loan, and if you didn�t file for a new title with your loan on it, you can�t enforce a repo (this is for Colorado, but it is similar for other states). It seems to be that you need to go to court, and prove to the court that you have a right to the car, depending on what is in that letter, and then the court can issue a repleven, which is a court order that can be enforced by law enforcement to get the car back. good luck.