If your customer's are in the same town and you do quality work then they will bring you their car. It's your job to educate your customer on what you're going to do for them and why you're the best choice as compared to their other options.
If you're going after daily drivers, like an Astro Van for a Soccer Mom or Dad then this type of customer probably won't bring you their car because they're not into their car.
If you're going after people that consider their car an extension of their personality then, for example a Mini Cooper owner, a Corvette owner, a Viper owner, a Mercedes-Benz owner, a BMW owner, a Lexus Owner, a Honda S2000 owner, a Ford F150 owner, etc., if these people are looking for a qualified person that they can trust to touch their baby then they will bring it to you and even arrange for someone to follow them to your house to give them a ride back to their house or to work.
Find a way to educate them is your job. It can be over the phone, (hard), or in person, (better), word-of-mouth is the best but you need to jump start your business to get mouths recommending you.
A business card that is about your customer and not you is a first step. A website with your services, qualifications, prices and examples of your work is also a tool you should use if you're in this for the long run.
If your car looks nice and you keep it clean and shiny all the time then consider putting some kind of signage on your car. Maybe the glass and not the paint.
Most all the cars I've detailed in my life I would categorize as "Special Interest" because they've never been just common daily drivers. They're not always exotics or antiques, but something that's important to the owner.
I'll post some tips on this subject to the best of my ability as I certainly will never claim to be the best sales or marketing guy in this field.