Poor customer service

On the 3rd of August I took my car to the car care clinic at the Silverton crossing, Pretoria to get my car serviced. A month later(on the 2nd of September) my car started giving me problems, the engine light was on. Because I wasn't near Silverton, my uncle offered to change the spark plugs but only managed to change 3/4(as they seemed to be the problem). On the 6th of September I went to the car care clinic to tell the manager about the spark plugs and he said they weren't faulty but offered to give me new one, I asked him to put in the new spark plugs but he refused and said my uncle should change them. Fast forward on the 23rd of October the spark plugs gave me a problem again and I went back to him, he checked the car and said everything was okay. Than on the 27th it gave me a problem again and I went back to him on the 28th, he started off by being rude to me and said I lost my guarantee the day my uncle changed the plugs. He started being loud, rude, unfair and didn't give me a chance to speak at all. I felt helpless, I cried. I asked him to check what was wrong with the car and that I don't want him to change anything because he said if he has to change something than he will charge me for it. But he ended up replacing to things in my car. The following day the engine light was on again and I went back to him and he said that the car was fine and if the engine light comes on again. I should switch off and back on again it will be fine. Today my car is parked at home and the spark plugs are giving me a problem again. I decided to call Westvaal Menlyn and ask if my car using any spark plugs or unquie ones and the said unquie spark plugs. I decided to check the invoice I received at car care clinic to see which spark plugs were used. All I could see was that the spark plugs used were valued at R40 each. I am scared to return to him and ask if he used to correct spark plugs but because I am a woman, I don't want to feel as weak as I was on that day.