Incorrect billing

I was wrongfully billed and handed over to debt collectors by Avis. In 2012 September I was added as an assistant driver as stated on the section of the contract where I signed. However upon return of the vehicle the bill was not settled by the account holder who rented out the vehicle using his banking details and all. I was surprised to receive a call from debt collectors (Creditworx) telling me that I owe avis for a vehicle rented out. The only thing I produced at avis was my drivers licence and the only part I signed on the contract was on the assistant driver section. So I have been listed on ITC as a non paying customer for 3 years which has torn my credit profile apart for an account that by law I am not responsible for.