Faulty tracking device and pro-rata

This company is unprofessional. They had installed a tracker in my car and the tracker was faulty I couldn't track my vehicle they also couldn't track it as I asked for a report of whereabouts of my vehicle they couldn't assist saying the tracker is faulty they unable to locate. They flagged it red and the technician only came the following week and I kept on calling in regards with this matter. The only time they called is when they wanted the pro-rata money,I then informed them that the device isn't functioning and I can't pay for a service that isn't being rendered as the device isn't functioning. They then said after the technician comes they will get a report to see if I tempered with it or if the fault was with the technician. The technician came a week after and informed me that the problem was with them there was a chip on the tracker itself that wasn't properly inserted. The tracker worked 4 days later money was debited that's not a problem as it's a monthly subscription. They then want to deduct that previous pro-rata and it's now increased even though the tracker wasn't functional. I explain to the accounts employee of car track of the prior conversations I had with his employees and to check the report from technicians. Now they threatening to suspend me from the services if I don't pay the pro-rata even though the monthly was debited.