Service customers at auto dealerships are intensely loyal -- but are older and spend less than customers of independent shops and chains, according to a recent survey. So dealers must win converts with online promotions and prepaid programs, said Mike Martinez, chief marketer at DMEautomotive, the marketing firm that commissioned the survey. Dealer loyalists, defined as those who spend the bulk of their service dollars at the dealership, are more than twice as likely to be 55 years old or older than people who typically get service at independent garages and service chains, the survey found. Many independent garages specialize in vehicles on their second and third owners, who often are younger than new-car buyers. And the old vehicles frequently need more repairs than new vehicles. The survey, of 4,000 random adults who had vehicles serviced in 2011, also looked at average annual spending by customers who were highly loyal to dealerships, independent garages or service chains...