Cartrack Unit on a wriiten off vehicle

We have a cartrack contract on a vehicle and last year December the car was involved in an accident and was written off. The querry we had with them was that since the car is no longer in our possession, how do we continue paying for monthly installments. The argument with them is we have agreed to the terms and condition of the contract which mentions we will pay for the unit on the car for 36 months. Having agreed to that, but there no question that was asked about a total loss of a car where one will have the optio to either agree or dissagree to such . Our contention is if such was provided(option) one would have decided otherwise. We are told that we are stuck to the end of t contract or pay a cancellation fee of R1700.