Playing from SD when connected via USB to a car?

Here’s a stupid issue that occurred to me when trying to play music through a car audio system with a clip jam connected via USB. Once connected to the car, the cj becomes useless, since the screen will just display “charging” and I have no access to the menu. All my music is on my SD card and the car audio system won’t even see its content and just play tracks that are in the cj memory.

Is there any way to solve this? I don’t know that’s more stupid, the cj locking itself or the car ignoring the SD!

Car stereos usually aren’t smart enough to handle devices with more than one storage medium per USB connection, so short of finding a rare car stereo that could handle that, I don’t think you can do anything.