Wait... car mechanics too? If you’re already billed for human labor cost on your invoice, what percentage do you tip on top of that? How is it shared between employees of the shop?
Assuming you go to the same garage, and that the mechs are not the owners... Your car is one of many at the garage. You may have particular expectations on time or how you like your settings (specific wheel alignment, etc) A bit of extra "appreciation" means your car may get fixed earlier, and come back with a few extra things adjusted without being on the bill, along with extra care to meet your specific asks.
Or so I was told. I saw it more when I was younger than I do today.
I've never tipped a car mechanic. They bill their labor rates anyways, tipping in such circumstance should never be expected.
Generally, most tipping is at restaurants and bars/pubs. Hairdresses/barbers/salons is also very common because of the close personal interaction with the worker.
Most of the time you tip at businesses that would be expected to employ low-education, or poorer or immigrant workers (and by extension, more easily abused workers.) Restaurants, salons, car washes, etc, all tend to employ poorer, less educated people, and also tend to be highly tip-based businesses. So for the most part you're expected to tip at businesses that traditionally abuse and underpay their employees.
