How to Protect the Car's Undercarriage?

Hi,

I will be installing an alarm system for a friend and he is really concerned about is protecting his car from someone tampering with the underside of his car,  i.e. cutting his brake lines, explosive devices, etc.  In your professional opinions what is the best way to get this type of protection?  I was going to install 2-3 DEI 508D proximity sensors underneath the car (I would make enclosures for them to protect them from the weather)?  Will this work?

Also on the 508D packaging it states that metal does not effect it's performance.  Does this mean that the sensor can penetrate through metal?  Or does it simply mean that installing it on or near a metal surface won't effect it's field of protection?

Thanks in advance,

Bogdan