Cartracker failing a client

I have a problem with cartrack my dad was using the cartrack on one of his cars that he bought for me to use as bolt and we ended up not agreeing on terms then he returned the car to the dealership,so the car tracker company started calling me cause my number was registered as i was the one using the car so i asked them to go remove the device from the car that my father returned to the dealership so they can install it on my new car that is on my name they failed to do that mind you i asked them to put the tracker on my name since my father no longer need it n didnt wana carry on paying it so they failed to go n fetch the device but they kept on debting on my account until i went to the bank to reverse the money after they debit,then they called me asking whats happening i explained to them that i asked them to go fetch the device and come install it on my new car but they didn’t do that but they wanted money so thats why i was reversing it after the debit order mind you am Using the car for Bolt so they took 8 month fully without helping me.so when i asked them to cancel the contract they demanded me to pay 3500 for cancellation fee i refused cause they were the one who failed me again now they keep on calling me saying i owe them R6250