I'd suggest something like this.
http://www.pacsafe.com/www/index.php...n=detail&id=18
Secure it to the seat frame or under the console and if the weapon is secured it's easy to get to and it's easy to secure. You don't want something where you have to disarm outside the vehicle in view of the public.
Granted, it's not theft proof, but if stashed well it's probable safe from a smash and grab. If the vehicle gets stolen (which is a lot more likely than needing a weapon for self defense) then yes, it's gone.