If your purchase contract for the car that you purchased says nothing about a gps system placed on the vehicle pending your payments on it nor there are no other documents that you signed with respect to this car allowing for the installation of the gps, you are not contractually obligated to have the dealership place such a device in the car you are making payments on.From what you have written, the dealership that you bought the car from cannot compel you to have such a device installed in the vehicle you are writing about.