I think that has very little to do with car electrical grounding, it is actually due to your air cond is blowing very dry air and causing your skin to generate much static charge than when you're outside the car (E.g. at home). When your skin has mach static charges and you try to touch on the body of car, you will see spark. The spark occurs is because those charges will flow back to battery's negative (Ground) in order to get balanced.

Meaning that if your skin doesn't have those static charges, you will not have problem touching any metal parts of the car. Do 1 experiment, do not turn on your air cond and drive for some time then touch metal part of your car and most probably you will feel less or no spark. Add osim air humidifier to your car perhaps haha.