The moment to ask
Right after the invoice is paid and the car drives well. Not at pickup while they are looking at the bill; not a month later. The paid-invoice message is the perfect place: 'Thanks for paying — if we did a good job, a review helps a small shop enormously: {link}.'
Make the link one tap
Get the direct review link from your Google Business Profile and use that, not a link to your listing. Every extra tap halves the number of people who finish.
Ask everyone, not just the happy ones
Cherry-picking reads as fake and Google notices patterns. A steady flow of honest reviews, with the occasional four-star and a polite reply from you, is worth more than a burst of fives.
Point the listing at your booking page
Your Google Business Profile's website field should go to a page with your services, prices, hours and a booking form. A reviewer's friend who clicks through should be able to book without ringing.
Reply to every review
Two sentences, by name, mentioning the car if you can. It shows the next reader a workshop that pays attention, and it is the only free marketing that compounds.