Sensor to detect vehicle occupants from external viewpoint?
No solution; 

I think that defies Physics as the attenuation of IR is high thru metal and being an excellent heat conductor tends to spread fast with external solar, engine and winter interior heat is often much stronger than the occupants.  It would be like trying to detect a 1ns pulse thru a 1 Hz LPF.  Glass would be like 10kHz filter.

A visual camera with multiple views and human head recognition might be possible but very low SNR.  A dummy could pass. 


  There are a hundred really"cool" uses of thermal imaging but impossible thru a car.  here a local comic Engineer has a great video.