This sounds very, very off ... SOMETHING has to be missing. Perhaps the brain trust you were riding with had done something with the guns earlier, and you were swept up in the net that caught them from some earlier incident?
Here is 12025(a):
12025. (a) A person is guilty of carrying a concealed firearm when
he or she does any of the following:
(1) Carries concealed within any vehicle which is under his or her
control or direction any pistol, revolver, or other firearm capable
of being concealed upon the person.
(2) Carries concealed upon his or her person any pistol, revolver,
or other firearm capable of being concealed upon the person.
(3) Causes to be carried concealed within any vehicle in which he
or she is an occupant any pistol, revolver, or other firearm capable
of being concealed upon the person.
These were full size shotguns? If so, then this section should be dismissed out of hand as shotguns are clearly not covered under 12025(a). And how were you in control of the vehicle???
Likewise, 12031(a)(1) could not apply:
12031. (a) (1) A person is guilty of carrying a loaded firearm when
he or she carries a loaded firearm on his or her person or in a
vehicle while in any public place or on any public street in an
incorporated city or in any public place or on any public street in a
prohibited area of unincorporated territory.
This might be possible, but against you? A passenger??
Neither charge really fits the circumstances you mentioned. If what you say is the complete, God's honest proof, you should not only get the charges dismissed out of hand at the preliminary hearing, but, you might also make a personnel complaint against the officers involved as they need some SERIOUS retraining. And if their supervisors did not catch this error in report review, then the supervision and review process needs some repair.
Just how did they come to search the trunk and find the weapons? Perhaps there is some clue as to what happened or what the officers were thinking in why they stopped and searched the car.