I upgraded my car stereo a couple years ago and gained a USB port. I tried using a normal 2-inch long flash drive, but it sticking out that far wasn't all that visually pleasing. The much better solution was a low-profile version, which is barely visible, protruding out only about 1/4-inch. My 16GB version, less than $10, holds about 150 hours of music, enough to handle a Forest-Gump-eon drive from Boston to Seattle, back to Boston and back to Seattle.
One watch out is to check your stereo's USB support. Mine only supported USB 1.2/2.0 and a USB 3.0 drive didn't work.