Tip #1: Don't wear white or light colored clothing, period. Service bays are just dirty by nature.
Tip #2: You'll get a more pleasant environment if you can do them between 8:30 and 10:30 AM. By nature, automotive service's "rush" times are between 11:30 and 1:30 and 4:00-6:00PM, the same as fast food. Typically, technicians won't be in such a rush to use the equipment you'll be photographing between 8:30 and 10:30.
Tip #3: If you have to photograph painted/stenciled-on lettering, just an FYI: the equipment will look nothing like the photographs in the guidelines. This is equipment that gets the crap beaten out of it daily.
I used to work for one of the companies that is doing a series of revealed photo audits across the country right now (5+ years ago), so these are a dream for me. I'm more than familiar with the layout of a shop bay and I love revealed photo audits.