best defence is an AR15 with a drum mag and short M203 grenade launcher and an alarm system that gives you enough warning to get to the window to use said assault rifle.
99.9% of car thieves don't have a shooter so you will easily overpower the thieving cunts. Plus the rare ones who are armed, are likely to be armed with a pistol, and an AR15 at 600 rounds per minute beats a drilled out deact-react special any day of the week
and it it all goes wrong, in the words of Tony Montana "say hello to my little friend" and let rip with the M203.