Rabbi Moshe Feinstein addresses this (Igros Moshe O"C vol. 5 letter 21:11 [translated]):
"It appears that....it is permitted to open a car door and take out what you need (as long as no light is activated thereby). It appears to be permissible to close the door afterwards as well. For a car is not muktzah because when it is parked in his property, the owner often stores things in it....even in a place where people would be surprised/dismayed to see him open a car door on Shabbos, if they won't see him do it, it is not forbidden to do so."
His reasoning would seem to be based on the principle (found in Shulchan Aruch Harav 308:22 and in Mishna Brurah 308:26) that when something that should be muktzah currently contains the type of non-muktzah item which is frequently stored there, the muktzah object becomes tafel to the non-muktzah item and can be moved.
So in your specific case that you want to prevent burglary, presumably there are items being stored in your car, which would render the car halachically non-muktzah. However, it must be noted that you may only open and close the door if **a)**no electrical activation will occur, and **b)**noone is looking.