Disappointing Results in a Spectacular manner

I am utterly disappointed with the level of service we have been receiving from Mercedes Benz-Wonderboom Dealership in Pretoria. It all began when the front passenger airbag began to malfunction,my Brother took it to the Dealership.the electrical component was replaced at a price of R9800 labor included, which we had to pay cash even though the car is still under maintenance plan. Nonetheless, It has continued to have the same problem to date, we keep on booking it only to be disappointed the next day sometimes the same day from the Dealership, as it continues to show the problem on the dashboard.Its really starting to be an annoying, daunting task to say the least. Can this issue be resolved as soon as possible as my Mother uses this car to go to work daily. It has already been booked 7 times this year alone! It boggles me how an esteemed brand like yourselves can deliver such disappointing results in a spectacular way.