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It's interesting that you say that the fly won't hit the back windshield. Why are you so certain? Do you speak from experience? Have you had a fly stuck in your car?
The fly will, indeed, not hit the back windshield (if it's on the back, isn't it just a window?) but it has little to do with cabin pressure or containment.
The fly won't hit the back windshield as long as the car is moving at a constant velocity. If you were to accelerate, then the fly would, indeed, hit the back windshield just as surely as you would had you not been sitting in your seat (with your seatbelt on, we'd hope).
Here's a possible history as to how this fly got into your moving car. You stepped into your car and the fly surreptitious slipped in with you. Before you start the car, it is moving at a constant velocity (zero), so everything is okay and no one is being squished against anything (well, gravity is pressing you against your seat, but that's not the point).
You start the car and you accelerate to a decent speed. While accelerating, you are being pressed against your seat. You can feel it yourself--the greater the acceleration, the greater the force is pressing down on you. The fly will also be "pushed" towards the back windshield and will hit it if it can't fly in the direction of the car and keep up with it. If you're accelerating slowly, the fly will simply fly (accelerate itself) in the direction of the car and be fine (most people don't go from 0 to 60 in 7.6 seconds). If, however, you accelerate like a race car driver, then the fly will will not be able to fly fast enough and, indeed, hit the back windshield.
After you've reached the speed limit, you stop accelerating and now cruise at a constant speed. You will no longer feel a force on you and neither will the fly (did it ever?). Everyone has already been pushed to this speed and no longer need pushing. Because the fly is moving at the same speed as the car, there's no reason for the fly to hit the back windshield. This is the situation that you described and this is the explanation.
A more detailed explanation is that the car is being pushed forward with a force and it is this force that accelerates the car. Now, of course, you must be accelerated with the car (eventually!) and this is typically done by having the seat of the car push you. That's why you feel the pressure against the seat.
The fly's situation is a little more complicated. Because the fly is flying, it must fly in the direction of the car in order to accelerate with the car or risk being accelerated by having the back windshield push it up to speed.
Despite what I had suggested earlier, the air in the car is a factor for the fly but it is not the major one. The fly will be pushed forward by the air but it can only be pushed as much as air friction can push it. This is much greater for a fly (relatively speaking) than it would be for us, since the fly is so damn light, but it is still only air friction, also known as drag. It is this and only this about the air that pushes the fly. The pressure is only relevant in how it affects air friction.
I hope people have learned something from this. I hope you made it through and I hope I made sense to you...
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