2004 Cherokee sensor

My friend whants me to install alarm with sensor that would protect the outside of the vihicle, so it would sound warning signal when somebody comes close to the vehicle. 

What sensor should I use?

How do you add that to the alarm when alarm has only shock sensor?

I would suggest NOT installing ANY prox. (microwave) sensor. The are affected by the density of the air. (clouds, fog, and humidity) Also extreamly dry air. You will adjust it ....it will work fine, then you get a hot dry day.....and you get ALL kinds of false alarms. Then in the fall and winter, you could take and climb into the car without setting off the prox. sensor. ALSO....people LOVE to mess with a prox. sensor...they walk bye, the car chirps....so, HEY, lets have some fun.

I would talk your friend out of the sensor.....just my opinion.