I have had a tracking unit installed almost over year ago and they installed a faulty unit

I have had a tracking unit installed almost over year ago and they installed a faulty unit. I have been paying for this unit and when they did bookings for arrival of technician, it's either they came on wrong days or did not pitch. My Father had a technician out for his new vehicle and they offered to assist and install a proper unit and then car track started with requesting payment. I told them this unit was faulty from day 1 hence i should not have to pay but they have debited me twice without my consent and numerous emails and calls responded to explaining the same and I'm now being bombarded with calls and emails. Sales consultant no help. I have lost trust in them and am at a point of wanting refunds for the faulty unit they installed that I have paid for all this time. Really unprofessional service.