This is what BC gives for instructions:
5 Easy Steps to a Basic Air Freshener
Brush or spray shape with water diluted food color, soap dye, or poster paint and allow it to dry thoroughly. (This is optional)
Apply fragrance by painting on with an inexpensive paint brush or using a pipette to drizzle it on. We don't recommend dunking or soaking, since over saturating the material with fragrance results in greasy fresheners and will require blotting, which wastes fragrance oil!
Punch a hole at the top with a small METAL office-type paper punch. Plastic scrapbook punches may break, due to the thickness of the material.
Cut a piece of string, elastic, thread, yarn, raffia or ribbon to the desired length, and thread it through hole at top, knotting ends securely.
Package completed air freshener by putting it in a flat cello bag, fold down the top, and staple or tape it closed. You can also use a label to seal it, or staple a fold-over header card or a business card to the top of the bag.
I kind of like what Sarah S said about spraying seats. I keep towels over my seats so I'd be spraying the towels, which is even better. And maybe when my truck warms up during the day, the scent will diffuse in the heat.