Jbearnolimits,

If you are afraid to do anything to it because of losing rankings (and while others may not agree), the tactics you mentioned may do just that.

Since it is a local business, I would focus on getting as many reviews for it as you can. I would also focus on making the website and the plus page rich with localized keywords. For the reviews, you could encourage the business owner to offer people a chance to win something if they fill out a review after having service done. Handing out fliers with invoices is a good tactic to bring attention to it.

For the localized keywords, use some onsite best practices to make sure their website is in the best position possible.
Another idea is to get the business owner to agree to or start a blog on their own and contribute to it regularly. This would allow them to target more keywords and auto repair is a great niche to provide useful info. They could do little tutorials on how to change your oil or how to tell if your brakes are bad or maybe how to do other small maintenance things. I think that would be a total hit online.

Here are some other ideas,

7 Ways To Promote Your Business Online For Free

Hope that helps,

Shawn

First of all, your site should have content and your site should be updated with relevant content regularly, it would help in ranking too, there is no doubt to it.

Apart from this, use some links from guest blogging, it is too helpful these days.

I have owned many domains and many sites... I have taken old sites that were buried in the serps and changed them and watched them rise, I have done the same to others and seen no changeat all. I have also had sites on the first page, changed them an seen them fall to the depths of serps hell. If you are on the first page I'd be very cautious on making any changes at all. Maybe a expert(if they exist) can guide you but from experience I'd leave it alone, except for making changes the the site actually needs. Building some more backlinks can't hurt, but be careful changing the content of the site in a effort to rise above the competition, you might just find yourself much worse off.

First of all add great content, and update your website if not daily, weekly.

Try to get links from Guest Posting on automobile related sites.... It will help you increase your rankings.

There is no need to update content regularly. This is an auto repair shop. That is just dumb.

A large part of local SEO is based on citations. You need to make sure they have a Google+ account for their business (formerly Google Places) and that it is filled out completely. Then you need to start creating citations for them. In most towns, if you do that with some very light link building, they will be #1 in no time.

Local Ranking Factors are here
First factor is Number of maximum Citations for that business
2nd is reviews
3rd is onpage
NAP should be OK on all places and same.

Let me know if you need Citations Submission service I provide this service on half price which people charging on WF @ full.