Mercedes Benz failure to fix their product & refusal to refund deposit

Mercedes Benz should actually be zero or negatively rated! I took my Merc C180, 2012 model , ~120000 mileage to Merc Centurion dealership for routine service. After they did a diagnostic on it, they gave a quote of R16000. After I gave green light to go ahead, Then they phoned me to say they had discovered some gearbox & water mixing problem. Their quote then skyrocketed to ~R250000!! After I challenged them on this, we were told this quarter of a million rand quote is worst scenario? They then said they would do something different, calling it a flush? We were then given a revised quote of R26000. After giving them the go ahead, they then demanded a R12000 deposit, saying they needed to.order parts from Germany! Few weeks later after we had paid the R12000 deposit, they phoned saying they were not able to solve the problem! Then they refused to refund my deposit, basically saying I should come collect the car! I then escalated this injustice to their Merc SA corporate head office who promised to investigate. Merc corporate office then came back & effectively sided with the dealership, saying also that the case is closed.