Invoicing without a contract

When i purchased my vehicle a year ago, i had the Netstar tracker installed. When they contact me regarding the fees, I declined to subscribe to the monthly fee. However, I keep on getting invoices and also been handed over to debt collectors for non payment. Numerous times i have requested from customer services to supply me with the contract for which they are billing me, but either they don't send it or tell me I must find it myself. Please sort this out ASAP, because i am not going to be nice if i get blacklisted or my credit rating gets harmed by your company's incompetance