I have seen Cohanim drive into cemeteries and not exit the car. Is the Tumah of graves not applicable to the Cohen if he is enclosed? May the car door be opened to allow a passenger to get out?

Better for a Cohen not to enter a cemetery, even in a car. One who does, can rely on the opinion Rav Eliezer Waldenberg writes in his responsa, Tzitz Eliezer 15, 42; that a Cohen can drive through a cemetery in a car. Kol Tuv