Unwanted car device services

I had a car contract tracking device with Tracker for 36 months.Contract was saying to call them to notify about the contract expiering 30 days prior and on the1st July 2015 did telephonically notify them.cancellation department was to call me and noone speaks on the other side but answering voice responds.now I asked for their cancellation contact details and they said they don't ha.Now they continue debiting my mony at the bank saying I'm now on month to month contract without my knowledge and initial contract that I had agreed to was 3 years which is long over.I feel harrased and betrayed to keep paying for the car that I even traded it a year and a half ago.Please assist.