Idk. There's usually a seal around the doors and at the bottom. Definitely don't forget the floor heater vent. You are going to have some loss in a vehicle. You need to generate more CO than whatever the loss is. People have used generators in cars before with success, but that's nasty having to breath all those noxious fumes. The only problem doing it like this is you won't know exactly how high a level of CO is in the vehicle. The lower the amount, the longer it will take to go unconscious. Myself, I have a CO analyzer, so I can measure the actual CO concentration. In your case I'd say that if you get in the vehicle and after 5 minutes, or so, nothing really seems like it's happening, I'd abandon the attempt and make the assumption that the level just isn't high enough.