unable to fix my car but still charge R22000

I am sending this email to complain about the bad service i have received from Eastgate Mercedes benz dealership ( Bedfordview). I took my car for fixing when it was showing red light battery in the screen. They change the whole computer system of the car and re-wiring it. I paid R22000 thinking it has been fixed but when i drove home same day, the car was still displaying the same problem in the screen. I was shocked and surprised that how can they give me the car back that was not fixed and charge me R22000. When i return it back they are now asking me to pay R10000 to replace the alternator. what i figure out is that they doing trial and error. They have replace the computer system which was not the problem because i had the same problem after paying the R22000. I need your help because i feel like i am being ********** here. My biggest surprise is why they cant locate the problem. I have already spend R22000 paying for things that was not the problem in the car. Currently the car is in their workshop and i dont have the cash to pay R10000 and i doubt if they would have solved the problem as well. My car registration is YSF 992GP.