Lightning is very powerful - powerful enough to rip right through a couple of inches of tire rubber.
However, you are still pretty safe in a car. The reason is that the metal shell of the car forms a Faraday cage, which keeps the lightning outside of the car, and you safe. As long as you are mostly surrounded by metal, the conductive metal will prevent the lightning from striking you. However, this effect probably would not occur for a soft top convertible.
Faraday cages are explained at wikipedia.