2003 Mitsubishi Lancer rear brake not working

HI. I have a 2003 lancer oz rally edition. When I bought the car, the previous owner modified the rear brakes from drums to discs. Now the rear brakes dont work(i think it's because it's worn out). I have taken it to the repair shop, but they said they cant fix it because they dont know what type of discs the previous owner installed. I have lost contact with the previous owner, I cant find out what type of disc brakes he installed. So can you help me to find out what type of brakes might fit there? Thanks