Your car has more smells than you can identify, starting from the moment it's born in the factory, when everything from the dashboard to the seats begins to outgas, forming what you know and love as That New Car Smell. If your car is used, you've likely gotten used to the myriad olfactory clues to the car's life before you -- but your guests sure as hell haven't. They'll smell every cigarette, every spill, every fast-food order's lingering scent as it gradually decays from sublime to sour.
You could use one of those two-buck spray bottles you find in your local parts store, or a clip-on scent dispenser in Spring Moon Orchid Blossom. But the factory-made, chemical-laden scent only covers up the odor. So if you don't want your ride smelling like an Uber, there's another way: the DIY way.