I am no longer a soft franchise for any product or company, all the tacky signs have been removed. I do have a couple up on the workshop front which we designed, promoting the services we offer, including "Bosch Diagnostics"
My customers come to me, because of who i am, not the brand names over my workshop door.
As for promoting your business, well word of mouth has always worked, and kept me in business for 25 years
One thing that has worked well recently is my website. I have had more calls from local people this year than ever before. This was because of two reasons
1) Make sure your site is set up for search engines (meta tags, description etc)for your local area. You want your site to attract local customers, not someone in Australia. Local content is king, also include a good clear map so they can print it off to get to you
2) Make sure you include all the services you carryout, and give some prices if possible, make the descriptions brief but honest
As you say, most customers have never heard or interested in brand names, they just want their car fixed by an honest garage. That honesty has to be earned and cultivated, not bought with a few suppliers signs