I believe a car is repaired like once per year on average and a customer would be looking for a repair shop only for two weeks in a year. That means that only 1/26 of the listeners will be "open" to your ad, the rest will just forget it. I would suggest a more direct way of approaching the customers with an active need. They search for 'car shop' on google - google ads, they may visit car parts stores - affiliate marketing. I would also suggest calling/e-mailing last year's customers to schedule a regular check-up.