This is related (but NOT duplicate) to http://feedback.arma3.com/view.php?id=5610
Motivation:
One might (rightly) consider this unrealistic, but the current implementation where you can not even access your own gear because of sitting on the commander's seat is no more realistic. Also the restrictions should be tied to your position, not to your role. A fully accurate inventory simulation is however unlikely to be added.
That being said, I would consider it much less annoying being able to access the car trunk from e.g. the gunners seat than having to stop and exit a vehicle just to reach into my pocket.