Car scenes often use camera placements that are both INT. and EXT., so INT./EXT. is usually appropriate for their scene headers.
INT./EXT. STEVE BLACK’S CADILLAC – DAY [DRIVING]
This is not a hard and fast rule. If your scene is obviously either INT. or EXT., use it. For example, if you have a movie about a family that has encountered a shrink ray, and your centimeter tall characters are adventuring from the back seat of a car to the front, your scene is probably strictly INT.