You can make a simple car diffuser from just a clothespin, hot glue and felted balls. Glue the balls onto your clothespin. Whenever your car smells musty, or you need some Stress Away as you drive, drip a drop or two of essential oils onto the balls and then clip it over your air vent. Easy!
GlennaC's cabling without a cable. DO THIS. You know that every time you tell yourself that of all the things you will do, THIS IS NOT ONE OF THEM, EVER, then you absolutely positively do it. So DO IT. And be glad.