Overcharge and refusing to fix my car

Bosch service analysis the vehicle and quoted an amount of R12000 to fix the vehicle. After 3 three months i can and paid R12000 and i was told the price to fix the vehicle is now R15000 and no explanation was given for the increase. I made an arrange that i will pay the remaining balance of R3000 once i come to collect the vehicle. On the day of collecting the vehicle i was told that a part was added on the quote and i now need to pay an additional R5000( this part was added without my pre-approval) meaning the overall cost for the vehicle ended to R20000. I had an huge fight but decided to pay the R20000. I made an arrangement that i can't afford that amount and made arrangement to pay the balance ended of the following month as i was never informed of the additional part which was already added. After this conversation during the month i received an sms charging me for storage which i than contact the dealership and asked why i was being charged and i never got any response from them. End of the month i collected my vehicle and paid the balance of R8500. After 2 weeks my car broke down again with same issue and they are refusing to fix the car stating i should meet them halfway