With $21 billion in overall ad spending, the automotive industry is still the largest U.S. advertising category. Where its ad dollars go often determines where others' marketing budgets follow. 

The automotive category, consisting of manufacturers, dealers and after-market vendors, will account for $2.69 billion, or nearly 14 percent, of the $19.5 billion estimated to be spent on online advertising and marketing this year, according to eMarketer.

Search marketing is the most effective component of the automotive category. Consumers are heavy users of search when online auto-shopping. The auto industry will spend $1.06 billion on search marketing alone this year and more than $1.7 billion in 2010, eMarketer estimates.

"Automakers and dealers pulled dollars out of television, newspaper and magazine media buys and redirected them to digital media in 2006, including rich media ads and microsites, search engine optimization and mobile marketing, and short films and video-on-demand," said Lisa Phillips, an eMarketer analyst.