I agree with John Sharp. Definetly a market (at least in my neck of the woods). I've been doing fee based detailing for about a year. I do no advertising at all and yet am able to keep as busy as I want to be (except for the nasty winter we just had). Do an exceptional job and people will want to go to you regardless. Once word gets out that your work is second to none they'll make your schedule work for them. At least that's what I've seen.
I do all my work on weekends or every other friday (I have two fridays off every month) and make it well known that I only detail by appointment and I never do more than one vehicle a day. That's how I want it. I've found people like knowing that I'm focusing on just their car and not trying to get done with it right away so I can move onto the next one. Also, don't try to be the least expensive guy in town. Oddly enough, often times people become skeptical if your prices seem too low. Do a great job, ask a fair price (maybe charge a little more and bill it as premium, by appointment detailing) and you'll be busy.
Sure, I love doing it and the extra cash is great but I still want to have tome for myself (fishing is #1) and of course, my family. Thus far I've been very happy with how things have turned out.
Good luck!
Natty