The temperature of the cars seats and the thermal energy of the seats INCREASES.
Sunlight can trasfer heat energy by radiation, when heat is transfered, the seats gain heat and the temperature rises due to the increase of kinetic energy of the molecules, the molecules move faster. As the temperature rises, the thermal energy is increased too because kinetic energy (and potential, but this is about the state of matter) of the molecules makes up the thermal energy of an object.