E320 Side Mirrors

I have had three side mirrors stolen from my 2000 E320 in as many months. I understand this is quite common in the metropolitan area of Los Angeles. Does anyone have any ideas about how to foil future thefts? Sometimes you can't avoid parking on the street.

Razor blades

There are some verbal warning security systems which you can install to the car.  If anyone gets close to the car, it broadcast a loud verbal message "....Back off now!".      Very useful but it does annoy the neighbours.

Nobody cares about the verbal warnings in la,the people ripping off the mirrors don't understand engish.