After reading this thread, I am almost tempted to try this squash business. But I may wait until I can sample it in person.
For years, I've been using something a friend of mine clued me in on. Bed, Bath, and Beyond has these paper sachet packets that they sell in 3-packs. I usually put one up front, one in the back seat, and one in the hatch area. They're probably intended to make closets & bathrooms smell nice, but they work like magic in the car. They're a little strong for the first week or two when new, but they last for 3-4 months easy (6 isn't unusual), and are available in a variety of scents. I tend to go for the vanilla. It's just lightly sweet, and I get a lot of compliments on it from passengers.