Passenger seat if no one else is with me (never buckled, tho LOL).
If it's a short distance and someone is with me, then, between the door and myself. (doors are always locked).
If we are going somewhere far, then the backseat, or on the floor, depending on who/what is back there.
It it's just a clutch or a wallet, the passenger must hold it, no matter the distance!
If someone else is driving, then, my lap or the backseat.