You may find over 530 liters of space in 300M having put down the back seat. Effective space design in Chrysler gives more opportunities. The figure was formed by Chrysler during designing the 2000 year models. In Chrysler 300M you get 530 liters of free space in the trunk. You may always put small shopping things in 300M trunk.
It’s better not to load more than Chrysler 300M can carry. It’s optimal for Chrysler in this model to have 530 liters cargo capacity. Check also if your luggage in 1999 Chrysler isn’t heavier than needed.