I sometimes write an essay to offer advice on German issues....so this is one of those. On this topic....it's simply a word or two of advice in handling German mechanics.
1. First, when you move in and get to know your neighbors or co-workers....one of the recommendations you want early on....is who they recommend for car repairs. In a small town, this matters. If someone has been using Huns, the local garage guy, for twenty years....it might be a pretty good recommendation.
2. Some folks gravitate over to older used cars....if you have a car that's less than eight years old...use the better replacement parts. Bosch makes good parts, and I recommend them. If the car is over fourteen years old.....don't worry much about what parts you end up with. The cheaper German parts will work good enough. Also, junk yards and junk-centers are a big deal in Germany. If you need a replacement steering column or a new door....you might want to ask about the local junk-center and if they can offer a cheap replacement situation.
3. Some private German mechanics will offer a drive-around car if your car was going to be in the shop for two or three days. Most will have some kind of cheap fee associated with it....maybe 10-Euro....maybe 20-Euro. My advice is to pay the charge....it'll end up being a decent car that the mechanic keeps in good condition and a better deal than a rental agency.
4. If your car is totally screwed up, and the German mechanic calls at the end of the day to say he needs to order parts....he'll have them by the morning of the second day. The problem is that it might go into the third day before he can get around to the issue. Be prepared for a problem like that. Older cars mean longer periods of mechanical repairs. You pay for your cheaper strategy.
5. If you have a connection to the military facilities....you have access to VAT forms, with the tax-discount. When you negotiate with the mechanic....this VAT-discount matters. A 1500-Euro bill....could be trimmed by twenty percent. Also, if you hint that you'd pay in cash.....the mechanic might grin and offer a ten-percent discount on the bill very quickly (provided you aren't playing the VAT-discount game). But don't anticipate a warranty deal if he accepts this cash under the table.
6. Yes, there are great reasons to go over to the big dealer for your repairs, if you have warranty tied to the vehicle. If you don't have warranty, and this is a simple repair....you don't want to pay 300-Euro for a 160-Euro repair that your local mechanic could handle easily. All of the Mercedes, Audi, Volkswagen, and BMW dealers will charge a minimum of twenty percent more on mechanical repairs. Anticipate that. Most local mechanics have the computer system and readily diagnose problems in five minutes.
7. If you are intending to drive from Germany to Spain for a vacation, or to Italy for a beach trip.....you might want to drop your car off with the local mechanic two weeks ahead of the trip and have them do a 10-minute glance at important things like oil leakage or brakes. You don't want to be in Spain and be told of a 1k Euro brake-job being required.
8. You buy tires from a German tire-center....not your local German mechanic. I realize that your local German mechanic will often come up and do this special deal with your standard yearly oil change/tune-up. Four new tires for 250-Euro....no changing fee....free balancing....etc. It sounds good....especially when he says he can make this happen with a ten-percent discount if you pay in cash. These are usually no-name brands and you might be lucky if you get a full year of use out of the tire. Always buy tires from an actual tire-center, and don't go cheap. When you are driving 180 kph.....you don't want to sit and worry about a blow-up with a cheap tire.
9. German mechanics can be brutally honest. If you have some cheap car that you simply hoped would last two years and has failed six months into your goal....your mechanic might stand there and tell you the repair will cost as much as the car has value (your original purchase price). Then he suggest in a direct way....it's time to part with the car (to the car-dump). I often recommend against the ultra-method of cars, and this cheap-strategy. But if the mechanic suggests that this a harsh and high-cost repair....don't be attached to the vehicle.
10. If you find a good German mechanic, who you trust....stick with them.