all cars cabin breathes.. most heavily is continental models..
so what happens, especially the e36 onwards.. the boot breather is located at the bumper, near the exhaust, the boot breather is also connected to the rear parcel shelf. so it is easy to shut your car door.
Also this for air movement during the car door is shut for long periods.. so when the car is idle, exhaust fumes dun move so fast cos they are heavy.. since also your car ac is on, doors shut..the outside air is suck in..so will the exhaust fumes..
Sealed both boot breather recently. Seems 2 work as there is no more petrol or exhaust fumes in the cabin.