I believe most head units in cars are too ‘dumb’ to see 2 different drives in 1 unit (internal memory & external SD card). It will see only the main drive in the device.
If you want to use an SD card in your car, I suggest getting a small micro-SD card reader to plug into the USB port. Alternatively, you could get a small flash drive like SanDisk’s Cruzer Fit & copy everything from both your player and the SD card onto it and just leave it plugged into the port in your car. That way, you don’t have to worry about re-charging the player or it getting stolen. The Fit drive is virtually undetectable, reliable and inexpensive. Plus it stores all your music, acting as a back-up in case you ever need to re-load your player for some reason.