So after the guy at a garage asked my brother 500 euros to fix the car, my father advised him to go to the Opel garage in my hometown, as they would know more about the car.
My brother did so, and they asked him for about 800 euros.
Not only that, they fixed something else without asking my brother, and charged him 70 euros for that.
A couple of days later, my brother took the car to a friend of a friend of his, who bought a part to replace in the car, and thus solved the problem. Total cost: 50 euros.
Another interesting fact about the official Opel garage in my hometown is that everytime my father took my car there, for the past 3 to 5 years, they would fix a problem, charge some money and... a few days later the car would start developing a new problem...
A friend of mine, from the same city, with another Opel, complains about the very same thing with the same garage...
Interesting, isn't it?