I really feel for the American automotive consumer. There are absolutely no industry standards in what must be one of the largest service industries.
I have told the story many times that if I were the Fugitive the safest place to work would be as a farm laborer or an automotive technician. There is a great segment of the industry that doesn't even need (or have) a social security number.
It takes so little to say you can repair cars that many people stay with the dealers for a semblance of legitimacy.
I have been involved with shops nationwide for years through a number groups: ASA, ASE, iATN, STS, IAIBMWSP, and as a BSC (Bosch Service Center). I would like to give my jaded opinion of clues to use in finding a good shop.
For 95% of your repairs and service, the best value will be with an independent. The reasons have to do with why they exist at all. They are usually the business result of a very good tech/manager. This means that technology will be a significant part of the highest manager. They are often family oriented and they usually draw most of their income from service NOT sales. At least in small cities, they can not stand to do poor work. I said 95% because, unfortunately the tools necessary to do the last 5% are beyond most small shops capabilities.
Now for some standards. I would start my search by looking for a BSC in the area. There are only 500 across the country. They were hand picked by Bosch from the cream of the industry. Many great shops didn't go with Bosch but all that did are special. I would verify that MB is a specialty (many BSCs only do MB but many only do BMW or Porsche, so check it out).
Look for AAA approved shops. That may seem silly, but the standards of AAA shops are REAL. To be a AAA shop one must pass a FBI background test, pass a visual inspection of all required equipment, and have certified techs in any area being worked on, and submit 100 consecutive work orders for a CSI (consumer satisfaction index) study which only 90 and above will pass.
Probably the best clues will be found from your friends and neighbors. Check out the various shops, interview them about anything but price, first. Ask their level of equipment specifically. This may require the type of knowledge easily attainable here. In other words if you have a 98 E320 they better have at least a CS2000 scanner or better if you have a light on. If they are doing front end work, do they have an alignment machine?
View the certifications. I wouldn't think too highly of any shop without an ASE Master Tech. This is a minimum lately, I would only take a driveability problem to a shop with a L1 certificate. This is a real certificate based on a hard test (only 25% of Masters passed the first time).
I will get off the soap box after one more comment. Please don't condemn a business through the results of a single event. In our business we write almost 10,000 invoices a year. I guarantee we will not do the best thing in every one of them AND I guarantee that if we were perfect we would still have people who thought we were wrong.
Bosch Master, ASE Master, L1
33 years MB technician