If you are thinking the Independent contractor route, David posted an excellent way to do it, that should very easily hold up to IRS scrutiny.
I'll answer a bit on the subcontractor vs employee. I have a degree in Finance, but work in accounting, so I know a lot more about this than I know about cleaning cars.
The difference between an independent contractor and employee is the degree of behavioral and financial control you have over them.
There is no clear line between the two. The IRS used to use a 20 factor test, to get a decent idea on their employment status, but that has since been discontinued.
Here is a link to the 20 factor test (its still a good general example of the differences,but as I said, no longer in use)
http://www.texasworkforce.org/ui/tax/forms/c8.pdf
The IRS has a form to determine if somebody is an independent contractor or employee. Its called an SS-8, and can be found on their website. The only problem is they don't really only want you using it to solve disputes, not hypothetical scenarios.
The way David does things, they supply their tools, they can come and go when they want, they can work for competing business (I would assume), they have the ability to make a profit or loss, so they are independent. Its a great way to save some money, but I would imagine it would be tough to always have correct staffing levels. I work a job as an independent contractor (unrelated to detailing), and if I feel like taking a month off, nobody can make me come in. If employee just said they were not coming in for a month, they would risk getting fired.
I have no experience with the detailing business, so i couldn't' tell you a fair rate. Paying but the job would motivate them to work efficiently, but (especially if they were not a professional), it would motivate them to cut corners. Whatever route you go, there are tons of programs that make payroll easy. I would sit down with a CPA and let them talk to you about it. Also how are you incorporated? Sole proprietorship? LLC? I'll admit that detailing isn't exactly dangerous work, but having an employee certainly opens you up to more liability, either through his/her neglect, or if they were in an accident. The fact that you have employees might mean you want to look into becoming an LLC if you are not already to limit your liability.
Obvious disclaimer, my advise is only worth what you paid for it (nothing). I don't know your exact scenario, state laws etc.