All you need to do is place the scented candle in your car's cup holder. The candle wax melts when exposed to heat — such as hot weather, or your car's heater in the winter — filling your car with your favorite scents. This works best if you have a candle that comes inside a glass jar because you don't want wax getting all over the cupholder. If you have a scented candle that doesn't come in a jar, just place it in a mason jar for the same effect.
— Read more in How To Turn A Leftover Scented Candle Into A Car Deodorizer at Slash Gear.
I’ve done this, it works! I have cup holders in the back seat which don’t get used. I put a small candle in the glass OUI yogurt jar in one. It scented the car just right, not to much. Easy to change scents and no mess. Besides a candle you could also use wax melts pellets in a small jar. I’ve not done that but it would seem feasible.
That’s so funny, I’d never thought of it.