In pure terms - doubtful. But then, in pure terms, no car really is. A true Faraday cage exhibits equal electrical potential at any/every point on it's surface which, when an electromagnetic field is applied, generates a current that causes displacement of charge inside the cage that cancels the applied field inside.
The glass in a sedan, and the cloth top of a convertible do not exhibit equal electrical potential as the steel body and so cars in general, while exhibiting somewhat of a Faraday Cage effect, are not true Faraday Cages.
For example, for a lightning strike near the car, passengers inside are unaffected. But, for lightning strikes directly to the car, the occupants are protected only so long as they are not in contact with any metal of the car.
Sedans offer the greatest protection while fibreglass and convertible cars offer less. You could probably increase the protection of a convertible if a metallic mesh were woven into the top material, but then, it would apply only when the top were closed. While theoretically, people exposed in a convertible are not protected, I am not aware of anyone ever being struck by lightning while driving a convertible.
