Indeed a PIR is for detecting motion from pets/humanoids (anything that emits enough infra red through heat). The P stands (kind of) for "Passive" meanings it does not emit IR but just sense if something is creating a difference in IR radiations received by 2 sensors embedded in one component. (I'm oversimplifying a bit - actually the P stands for Pyroelectric Infrared Sensor but it's an easy way to remember what they do)
if you want to detect non "human" intrusion, you would need to find a RIR (reflective infrared) sensor which does emit IR and measure what's coming back - so can detect stationary and moving objects. in arduino space most are sold as line detectors, you don't want that because they sense only a short 1.5 cm away. You need something stronger. Note that because they emit IR lights, the "lens" needs to be clean. if your car gets dirty, that might affect the performance.
you could try to play around with this extra-long-range SHARP distance sensor and measure on the Analog out any significant change happening. (because it will return the distance of the seats or whatever is in the car - but that signal should stay stable as long as nothing comes changing the configuration of the car once it's activated).