OK, you seem determined to do this, so...

Leave the door of your car unlocked.
Your hands might get cold enough that your find it really hard to turn a key in a lock. I have seen people that near to frozen and have been there once myself.
(besides, if anybody shows up intending to break into your car they will just bust a window out whether it is locked up or not)

If you start shivering end the experiment right then, that is the first stage of freezing to death.