If you recive the newspaper,have milk delivered or any other real early morning deliveries,look at those people. Also, kids in the neighborhood.
The kind of person who breaks into cars are kids. Adults dont do it unless its to steal the car/truck. Kids do it as its an easy crime,they only get thier hands slapped for it. I might even talk to a cop and discuss them talking to any up and comming new criminal kids in the neighborhood. Only a kid looking to climb the criminal ladder is going to take a weapon from a car.
I have some older Ih trucks and some dont lock. When I lived in Arvada,about every 2-3 months,I would find it gone through and usually a door wide open(hard to close) I knew a local kid was doing it but proving it was a different story. Consider baiting the car with an empty cd case book left out in the open and a camera on the car. I bet you get lucky after a few nights.