From what I have been told, ammo must be in a separate compartment in the vehicle that is not accessible to the driver or must be in a separate locked case.
If you have a trunk, the ammo should be in the trunk. A locked glove box may suffice. If you have a pickup, a locked case would suffice.
Interestingly, the law from the PDF below is, IMO, not clear whether the firearm and ammunition must be stored, readily inaccessible and unloaded, separately of each other during transport.
18 U.S.C. § 926A. Interstate transportation of firearms
Notwithstanding any other provision of any law or any rule or regulation of a State or any political
subdivision thereof, any person who is not otherwise prohibited by this chapter from transporting,
shipping, or receiving a firearm shall be entitled to transport a firearm for any lawful purpose from any
place where he may lawfully possess and carry such firearm to any other place where he may lawfully
possess and carry such firearm if, during such transportation the firearm is unloaded, and neither the
firearm nor any ammunition being transported is readily accessible or is directly accessible from the
passenger compartment of such transporting vehicle: Provided, That in the case of a vehicle without a
compartment separate from the driver's compartment the firearm or ammunition shall be contained in a
locked container other than the glove compartment or console.