The next step is to do a second sort on the box of keepers. If the weather is too hot or cold then bring the box(es) of keepers into the house where it is more comfortable. Further sort these things into: 1) stuff to put back into the vehicle, 2) things to put away somewhere in the home, 3) items to give away, and 4) trash.
Lastly, deal with the separate piles. This is the most difficult step for many folks and it is often neglected as energy wanes. This is precisely why I suggest tackling one small section of the vehicle at a time – to make putting away less burdensome. How to deal? Replace the things meant to stay in the vehicle. Return household items to their rightful places. Bring the giveaways to their new homes. Put the trash in a bin.
Admire your clutter free car and congratulate yourself for a job (or part) well done. Enjoy the pleasure and lightness of driving your newly decluttered car.