BlueJ: Did they then refund what you payed them?
Looking at this from outside, I see this:
1: You have a payment deal.
2: You honor the deal and pay monthly.
3: They receive the money.
4: They change some internal rules on their own, and still take your money.
So in short, you honor the agreement, by completely fulfilling it's term by payment. There is no outstanding debt, since you have paid to the point.
I do not know your court system, but where I live, they would loose at court. It should be in your case too, since you did fulfill the obligations of the contract that you SIGNED. You have the receipts, so hire a confiscator, give him the paperwork, and send him for the car
Better still, go public, talk to the press, and use the sailor mouth subtly. Because if this is their working method in a case that simple, you are not the only one, and the simplicity of the whole thing makes a nice fat slice for the media.
Best of luck, and kick'em hard