You may WANT them to have a few minutes driving time, so if you're
carjacked they get far enough away to not be a threat.  Also, if you
have a mid-eighties or better BMW, hook the PIC up to the double locking
switch in the driver's door.  That way the windows and sunroof close,
and they can't unlock things from inside.  Of course, it will hopefully
be 120 degrees on a nice sunny day in the desert when they steal you
car, that way they have a good time...

  Also, do an altavista search for the BMW faq.  The section on alarms
has some excellent ideas on how to install things so it can't be
disabled easily by a professional thief.  Like sticking the high pitch
inside screamers down the air vents and etc..