In other words, this was not a "lemon law" claim. You personally believed the car to be a lemon, and you chose to let the vehicle be repossessed. From what you say, the debt has been sold to a debt collection firm. They are having difficulty providing you with the documentation you requested, most likely because they don't possess that information (it's in the hands of the original creditor).
I suspect that the statute of limitations on the debt was four years, and thus that the debt is probably no longer collectable. Take a look at the
sample dispute letter in this thread.