No answers yet?? Come on, people...
Most of the big-name niche perfume companies (and many of the designer ones) do candles, too. Creed, L'Artisan, Diptyque, Bond No. 9, Prada, Armani, Acqua di Parma, etc.
Some do room sprays, too (Creed and Diptyque spring to mind). Diptyque sells some of their scents as oils. I don't know of any big fragrance companies doing those reed diffusers, though, but there's a sort of backwards thing where some of the better diffuser companies (Agraria, DL & Co, Dr. Vranjes) have started making perfumes.
I've never encountered any sort of fancy, high-end car scents, except for Frederic Malle's "rubber incense", which is basically a rubber rectangle that looks like a mouse pad that's heavily scented. At the Frederic Malle store, they suggested that it would be good in a car's glove compartment (though it's Malle so it was really expensive, of course).
You'd be better off picking a bottle of something you don't like wearing any more and just keeping it in your car (but don't pick anything you love too much, because the heat will kill the scent in the bottle after a while).