I would have to presume your payments have simply amounted to interest payments. If so, there is no reason they would be required to forgive the principle. It's kind of a built in income for them the way it is.
If the vehicle is worth less than the owed amount, you might ask them if they will accept a lesser payment as payment in full. If they repo it, they will only get scrap value or market value which. If you would offer scrap value or market value, they may accept the lower payoff since that is all they would get if you turned it over to them.

