Cartrack failed to install a tracking device on my vehicle but keep on taking my money monthly

In 2018 i bought a vehicle which i installed a tracking device with cartrack, i bought another vehicle in June 2020 before the contract expired and asked them to come and install in the new vehicle, they told me there is a fee that i need to pay which i never had a problem with. They agreed to come and scheduled an appointment, but they never showed up on the scheduled date, I called them and they scheduled another appointment which they failed to honour again. 1 week passed and i called again they promised to come, another week passed but still not coming, i called and explained to them that i cannot drive a car without a tracking device because i am living in a high risk area, and the insurance company needed a fitment certificate too. They scheduled a third appointment but still didn't come or communicate, so on the fourth week I decided to look for another tracking company which came the following day and installed the device. i stopped calling them and they never even a single day contacted me to apologize or to ask if they can still come. but at month end they deducted money which I reversed since they failed to come and install the device. Apparently, they went to the dealer where I sold my old vehicle to and removed the old device but never came to install it on my new vehicle. every month they keep deducting and i keep reversing, i tried to stop them from deducting but they are telling me that i must cancel my contract which i need to pay over R2000 or continue paying my monthly premium even though i don't have a tracking device. They are being so arrogant and bullying me everytime i call to ask them to stop taking money from my account. They have sent me a letter of demand from the lawyers and I have told them that they failed me and I cannot pay for the service which I am not receiving.is it my fault that they failed to come and install the tracking device?