How does the chassis ground on an automobile protect against lightning
Tests** have shown safe results with an occupant inside a car exposed to a 1 million volt discharge but there will be exceptional situations.  (e.g. open sunroof and with roof antenna.)

Those tests**, which you may search on Youtube, were even performed on a dry car. A wet car makes it easier for the >1kV/m E field stress as water is 80x stronger as a capacitor to conduct the charge to ground.  It is minute contaminants in the medium that cause the arc to bifurcate and change direction slightly or "fork" in the general direction of opposite polarity.