Cars aren't sealed at all, they have vents usually hidden behind the plastic panels in the trunk that are kind of one-way only, the little doors in the vents blow open when you shut your car door to release the pressure and prevent your eardrums from blowing. Even though they're one-way-ish, there's not much of a seal, on top of that, the HVAC system isn't really sealed. The thing most people don't realize is that even though you wouldn't be increasing the pressure all that much by releasing a gas, you've still got craploads of oxygen inside to flow out. However, the partial pressure of oxygen will likely be a whole lot more than the pressure differential increase you would be trying to replace the air inside the car with, meaning oxygen is going to keep flowing back in by osmosis (high to low concentration) to equalize the concentration with the air outside the car. That force of osmosis is probably just going to win every time, even though it's only about 1/5th of the total pressure of the ambient air (14.7psi at sea level, so depending on concentration difference up to 1/5 of that 14.7 is going to be trying to diffuse back into the car and there isn't a whole lot you can do to stop it).
**Edit**
I should add that it probably could work if you had a really high flow of inert gas, or if you had some sort of apparatus like the bag with a small volume inside flowing out at a high rate so that the oxygen doesn't have time to flow back in. I just don't think that it would be feasible to have enough flow/volume/reserve of helium unless you had like a large welding bottle because the volume of the car is huge compared to something you could wear. I would be afraid of running out of the inert gas too early. Even just a little bit of oxygen is too much, makes a huge risk of failure and potentially permanent injury.