If you put a few drops of your favorite essential oil on a clothespin, the wood will absorb the lovely scent. When you put the clothespin into your car’s air vent, the smell will be released into the whole vehicle
So for the trunk shelf, not sure what the pin is suggesting and this may be an expensive way but try this idea. You can buy rigid insulating foam from Home Depot...$10.. http://www.homedepot.com/p/Cellofoam-3-4-in-x-4-ft..., cut it to shape (It's a 4x8 sheet, could make 2).
I use a plastic shoe box as my trash can in the car. It is pushed under the front seat by the back seat passenger. Also, when the kids need to eat crumbly things in the car, they eat with it on their laps and crumbs fall in. When they are done, the lid is put on.