Unhappy Client

I bought a Mahindra XUV500 at the garage in Boksburg and I am staying at Kinross. The car was well maintained. I then took upon me that the car will be serviced at Mahindra Bethal since is near. I have a major problem with this place. In May 2021 my husband took the car for Service quotation to them and when he was going out the car showed a fault by all windows of the car going down and doesn't go up. They told him is the ECU and it will cost R16888.33 which he paid on spot. They told him they are ordering the part. The car has a warranty with Absa Utimate Warranty under Innovation Group. It took approximate two weeks for the part to be delivered as the warranty said they are not covering because the fault is not as per the Proforma Invoice that my husband paid. I started to interfere and getting to the bottom the fault was the body computer which was less the amount they charged and we got refund after i took upon me to get clarity as they took advantage that my husband doesn't understand English. Now I took the car for Service on the 29 July 2021. On the 14th August 2021, we started the cat as normal and it responded and after few minutes it stopped. My husband open the bonnet and there was nothing showing as fault. We then closed the car and tried it again in the evening and it was not showing any signal and front windows went down. I took a breakdown on Monday (16/08/2021) at work and took the car to them and on arrival I told them what it did. They said they shall look at it. I didn't hear from them until I called them on Thursday 19/08/2021 to request clarity and the response was that they will inform me should anything happen. On Friday they called my husband to ask him to sign papers that they are thinking the problem is in the engine so they need to open it and he will pay for fixing. my husband called me and told me while he was there and I spoke to them and told them that he won't sign for anything they have to check and claim since the car is under warranty. The guy explained that one client bought the whole engine because the car was giving him problems. I told them that I won't agree to anything until I get full report on what's the matter with the car. Since then I haven't received the feedback from them even after I call their Head Office to complain no response or a call to inform me about the progress.