One of my friends has the one of the usual annoyingly loud car alarm
devices, the kind with an external speaker.  He put the 135dbA speaker
inside the car.  It is virtually impossible to stay in the car with the
alarm at full chat, unless one is deaf or soon to be.  With the doors closed
the alarm sound is noticeable at some distance but not bloody annoyingly
loud.  After a period of time shredding the eardrums of the mice that may
have hooked a ride on your last camping excursion the alarm resets and waits
patiently like a cobra for the unwary to return.  To get an idea of what it
sounds like, find someone with an alarm system, put your ear next to the
speaker and have them set it off.

Doesn't stop the break in by a determined thief but does dissuade further
theft of property or further damage.