28-1212. Presence of firearm in motor vehicle; prima facie evidence.
The presence in a motor vehicle other than a public vehicle of any firearm or instrument referred to in section 28-1203, 28-1206, 28-1207, or 28-1212.03 shall be prima facie evidence that it is in the possession of and is carried by all persons occupying such motor vehicle at the time such firearm or instrument is found, except that this section shall not be applicable if such firearm or instrument is found upon the person of one of the occupants therein.
So, yes, if it is in the vehicle it is deemed to be in the possession of every person in the vehicle. The only way it can be claimed to be in the possession of just one person is for it to be physically on the person claiming it as being the one possessing it. This also works the other way...if your buddy has a gun in the car under one of the seats, you can be charged with possessing it as well.