Bad service and bad attitude

My dad bought a Renault Sandero in 2016. From Imperial select Germiston. The car was working perfectly fine untill a few weeks ago when he put this venter trailer plug in. The next morning the car did not want to start. So we called the road side assistant to come and start the car. They towed the car to the workshop to check what was faulty. The main relay fuse that blew. And they also said that there was a wrong alarm system installed. And the car is still under warranty. And now Mccarthy accused us that we installed the wrong alarm system but in the mean time it was imperial select. And now they refusing to fix the car due to cost of the company. But they are very quick to claim against the maintaince and warranty but u have to have first argue with them to get service.