There's no pat answer since your budget and the tactics you choose will depend on the geographic market and the cost of media there, the target audience you need to reach, the number of competitors and other elements, such as your dealership's reputation.
TV used to be the primary tool for dealerships and, in many ways, remains top dog today. However, now many spots are designed to drive customers to the web, and you may do well to examine how you can best use the internet to market.
Question added to topic Marketing • August 8, 2008
What's the BEST use of my advertising budget as a retail auto dealer?
As a retail auto dealer, what percentage of my advertising budget should I devote to various media, such as the web, newspapers, magazines, mailers, etc.