Damage unit installed in my car

Tracker installed tracking unit in my car July 2014 since then they have been debiting monthly instalment in my account for a unit that is not working. I phoned them last week Monday to check if the unit is working they advised that they will get back to within 2 working days which they did not I phoned them yesterday for feedback which to my surprise they told me that they need to send a technician to check the unit because it looks like there is a fault with it of which the lady that was assisting me told me that they have to many queries to deal with I have to be patient maybe they will contact me next week Monday. I am not happy with the way they are treating me, cause every month they are taking money from me for a service which I am not getting. I really need clarity on this matter.