No longer satisfied to have "just" a Mercedes or a BMW or a Lexus, more and more customers are adding thousands of dollars in custom parts to their new cars, according to the Dallas Morning News. Dealers are happy to oblige the estimated 15 percent to 30 percent of new-vehicle customers who want personalized parts and pieces.

Personalization is a new and expanding revenue source that is already generating $1 million or more a year in incremental sales at some dealerships.

For dealers, the trend is gathering speed at a good time. A slump is predicted in new-vehicle sales this year, and some dealers may need an extra source of revenue, industry experts say. "Service and parts, as a percentage of dealership profit, soar in periods when you have low profits or no profits" on new-car sales, said Paul Taylor, chief economist at the National Automobile Dealers Association (NADA).

"Sales are probably going to slow way down this year, and [service and parts] are going to have to carry more of the load," Taylor said.