Just wrap the insides of the car with electrically grounded foil blankets. Done and done.

"Microwaves cannot easily penetrate metal, especially grounded metal such as the electrically grounded foil blankets. The reason why emergency blankets are recommended over Reynolds aluminum foil is that these blankets are larger, more durable, and easier to hang up. Though some signal still gets through, and foil does not block pure ELF waves (ones without a higher frequency carrier), this is one step to greatly reducing the level of incoming microwave radiation."