Survey shows other shops surpassed them for the first time as the service pie grows.
Supply hits 20-month high in November as SAAR falls from October, NADA says.
Wholesale price depreciation picked up last week in both the car and truck segments.
GM says popular brand cultivates loyalty, brings in new customers.
Average for new and used vehicles decreased in the third quarter.
The nods, topped by best new model - the Toyota Prius - are based on analysis of vehicle-related data, including pricing, consumer reviews and ratings, five-year cost-to-own information.