TRANSPORTATION OF REPLICA FIREARMS
13. An individual may transport a replica firearm only if
(a) when the vehicle in which it is being transported is equipped with a trunk or similar compartment that can be securely locked, the replica firearm is in that trunk or compartment and the trunk or compartment is securely locked; and
(b) when the vehicle in which it is being transported is not equipped with a trunk or similar compartment that can be securely locked, the replica firearm is not visible from outside the vehicle and the vehicle, or the part of the vehicle that contains the replica firearm, is securely locked.
So if you got a trunk, and can fit it in there with a case and succesfully lock the trunk, there yah go.
If not it can go in your back seat, but must be out of the eyes from anyone outside of the vehicle and anyone inside of the vehicle. So in a gun case, preferrably locked case. And even toss a blanket over for extra precausion.
Plus if it's going to be left in the car, make sure you lock your doors. But I dont see why anyone would leave an airsoft gun in their car. Just sets the grounds for something really bad happening, and robbery.