How is a car seat purified from the impurity of urine in the event of repeated urination on the seat and uncertainty about it being sufficiently immersed in water to remove the impurity? And would taking it to a car cleaning shop to remove the seats and spray water into them remove the impurity?

It is sufficient to immerse the impurity in water until it is most likely that it has been removed. It is not necessary to wash the inner part of the toilet seat if you are not certain that the impurity has reached it. The persistence of the smell is an indication of the persistence of the impurity, except for the one afflicted with obsessive doubts, who should not pay attention to such uncertainties. If a trustworthy Muslim informs you that it has been removed, then believe him.