If the Shotgunner exits the vehicle, Shotgun becomes available to the remaining passengers as soon as the existing Shotgun rider has opened the door to exit.
Once Shotgun is available, you must call Shotgun before the other occupants. In this scenario, it is common for the other occupants to all call Shotgun at the time. If this happens, a quick game of Rock, Paper, Scissors, Lizard, Spock is often the easiest way to solve the problem.
2.11.1 If the Shotgun rider abandons his seat to do a Deed for the driver, such as purchasing Sunflower Seeds or pumping gas, then that person retains Shotgun.