Whoever sold/delivered the car/warranty/service plan/loan to you sells your information to these sketchy companies. So they know where you live and when you bought the car and which car you have. From that they deduce when your original warranty is likely to expire and send you the scam letter.

In this case, since the author didn't actually buy the car, maybe he took it in to get it serviced and THEY sold his name, address, car make to the scamsters (or something like that).