What you shouldn't do is sign a new agreement that allows them to charge you 18%, that is crazy!
If you are willing to have a repossession on your record then simply wait for them to take the car. You have already informed them that you can't pay and have offered it. They don't want the car though, they want your money. Their legal option is to repossess the vehicle. If they do then you will still likely owe them some money, unless you owe less than the value of the car.
Filing bankruptcy is a personal choice for you to make and there are a lot more factors to consider than just car payments. A bankruptcy can provide you a lot of protections, but certainly has its downside as well. You may want to consult a financial advisor to determine what is best for you and your particular situation.