Actually, this brings up a good question.    I have lived in several
different towns, and in each one there seemed to be a specific "harm
reduction" policy to follow when it came to auto breakins.   In some
neighborhoods, an alarm system would not deter the theif, and might
actually encourage them to check out your car further as you might
have something worth stealing.  In others, you are better off leaving
the vehicle unlocked, as the local thieves only want your spare
change, and are willing to break a window to get it.