If it’s a sale by description then the goods must be as described. Also, goods must be fit for purpose – and what is fit for purpose depends on the particulars of the sale ie a private individual purchasing from an advert may reasonably expect a car that runs without obvious problems unless they were pointed out beforehand.
As to the actual misdescription, what is the actual loss you suffered? CD Multichanger is fairly easy to quantify, but one extra owner isn’t.
I would look to the running like a nail issue and not the CD/extra owner if you want to reject the car.
What sort of car? How old? Main dealer or camel coated indie?