There are two schools of thought on this. First - turning something you enjoy into some extra income is a terrific solution.
The other...well, I hate to sound like a pessimist, so I promise to keep this short.

Doing something you enjoy for money can quickly sap all the enjoyment out of it if the wrong circumstances crop up. I do a number of cars for friends and family, but I never charge. The main reason is that I don't want to become obligated. "Well you did it last time, why can't you fit me in now," or, "It's not beading like it was a week ago - you must've done something wrong." Yada yada.
I'm an IT guy by trade, and this is the exact reason I don't work on computers or networks on the side. Likewise, my main passion is music. Back in my late teens and 20's I took any gig I could because I needed the money. The reason music is still a passion for me...and I know this is a stretch, but I'm convinced it's one reason I enjoy detailing so much...is that I now do it on my terms. I turn down jobs I don't want, and can fully commit myself to and genuinely enjoy the ones I do accept.
Don't get me wrong - I completely respect the folks that do this professionally, part-time or full-time. I just think of it as a pleasure, and you know what can be said of mixing that with business.
With respect to marketing, have you considered ads in local independent papers, or perhaps craigslist or some other online posting if there's an offering in your area? Good luck!