If it were me...
...well, in my jurisdiction, I'd be pretty confident that no criminal charges would hold up against me, but that's partly because of the framing of the theft provisions under the Criminal Code of Canada. (Theft is when an object is taken or withheld 'without colour of right'. That sort of arrangement would give significant equitable rights in the car.) Of course, I would also have paid by cheque.
Even so, I'd probably just return the car and sue the parents. There would be evidentiary problems, obviously, but even if I had paid cash, I should be able to provide a number of corroborative pieces of evidence:
(1) Significant withdrawals of money from my bank account at particular intervals. If I'm saying that I'm paying $440 a month, and I can show that around the end of every month I withdrew $500...easily done.
(2) When I service my vehicle, the mechanic takes *my* name. Even if I hadn't kept the receipt, I'd be able to get the mechanic's records to prove that it was me servicing the vehicle at that time.
(3) If properly insured, the insurance policy should be in my name, or at least should have my name in it.
(4) Once civil proceedings began, it should be possible to subpoena the parents' bank records, which would - in the normal course of things - show regular receipt of $440.