If you don't have a plan to contain these items, every time you go to place your groceries into your trunk you will be greeted with chaos leaving you no clear area to plop your newly purchased items.
Here is one quick organizing solution for getting your items organized in your car trunk:
(Now, I never said "Junk," nor as a professional organizer would I refer to your items as such.)
With your new plan to keep your daily trunk items in one place, you now have lots of free space for all your shopping needs.
What is the oddest thing you found in your trunk?