If hot boxing a car over an hour consumes enough oxygen to matter it wouldn't be possible to sleep in a car without running out of oxygen. A couple blunts doesn't consume that much.
My point is that the oxygen consumed over the duration of hotboxing a car is so trivial compared to spending hours sleeping in a car that if the former has any effect the latter should too.
Automobile passenger compartments are vented, otherwise there'd be problematic pressure spikes on door/hatch closures. You're not getting deprived of oxygen sleeping in a vehicle.