How can you be a manager of one person? Sounds like a really small department. The title sounds impressive, but there isn't the volume at that place to back it up. If you make $200 bucks a week then I would say you are doing very well in that place. You can make $200 bucks a day as your own boss in this industry. I made $280 last week in my spare time and I don't have much spare time. Between a full time job and a family I don't get to detail as much as I used to.
The title is impressive, and if they are going to put you in charge of ordering supplies, scheduling appointments, hiring, training new employees, and all that fun stuff, then I can see it being good experinece to get. If this is the field you want to work in and you want experience in running a shop then I say go for it. You can't buy experinece like that on a resume. Then again... which is more impressive do you think...? Saying that you managed a small dealership detail shop or saying that you ran your own sucessful business? Its something to consider. I'd take the job for the experience at least. You won't make your fortune doing it, but you can still detail in your spare time for extra $$$. Managing the shop would just be your day job.