I agree with the whole blaming the roommate because of previous incidents. But you still have to give him the benefit of the doubt.
It is kind of funny, how it happened at one of the only few times im sure your car was left unlocked. Cause I just dont see somebody waiting for you to just "accidently" leave there car unlocked to jack there stuff, if they want it... there gonna get it.
Or it could of been a coincidence you get robbed the only time you leave it unlocked.. get fingerprints.
You could bluff him, say they pulled a few fingerprints off the car and if the roomie didnt do it, then he wouldnt mind giving you his fingerprints, and the eviction process will be stopped.
if he holds out.... guilty, if he doesnt... he didnt do it...
No guilty party would let you get their fingerprints for free.