Step 6: P.s.
It's been a couple weeks now since I put these in all the cars and it seems to have worked to a certain extent. The moth balls disintegrate more quickly in the cars that get used more often so this design would be better for a car that is mostly just sitting.
Also, I noticed that is says on the box not to breathe the fumes. I don't know how dangerous it could be but the smell is pretty strong and you probably don't want to be breathing it all the time. As long as the A/C or fan is not on while driving there should not be too much getting into the cabin.