I found a datasource, it's dated (2011) it shows people trust their "shop" which could be a dealer, chain, or independent. With a 37% split going to independent and 30% to dealerships. I think you are right that we probably wont find any data on if people have a trusted mechanic since this also shows that there is a lot of price shopping going on.

But with all that said, in the broader conversation I'd say I was wrong. Even if you don't have a trusted independent mechanic more people prefer that option, even if they use a different one every time, over the dealership.

http://www.consumerreports.org/cro/news/2011/12/survey-consu...