Rockbox is all well and good until you realize your car’s stereo doesn’t recognize your iPod Classic as a music player anymore. Hours of forum rabbit holes later: turns out it’s special firnware that Rockbox can’t access, and probably won’t ever be able to (unless apple voluntarily forfeits some encryption keys).
Like every other DAP, Rockbox gets your iPod recognized as a usb storage device by most vehicles. For the stereos I’ve used, this is a headache because they needed to index the entire filesystem at an agonizingly slow bandwidth… every time I started the car or had unplugged the iPod. One car slowly gave access to songs as they were discovered, gradually revealing the files to you over time, the other just displayed a message until completed (which hasn’t ever happened, even on hour+ car rides). The same things occured with my Onkyo DP-X1, except that indexing would finish within my lifetime.
It was easiest just to use the 3.5mm jack for the car stereo tho, due to the indexing being required every goddamn time the car turned off or the player got unplugged. Which is a shame, bc my stereo may be stock, but they are quite nice and expensive DAPs (i even tricked the iPod out with an iFlash for SD card capabilities and a custom shell) and should be able to stream digital audio over USB.