The Sansa disables manual control when it is plugged in to a data connection. With your car radio head unit, you may have several options.
Depending upon the level of sophistication of the head unit, it’s possible to play files that are loaded on the device by reading them from the memory directly, or some units allow transfer from the device to the head unit’s internal hard drive. This may be limited, as most units may understand only mp3 format media, or may only be capable of MSC transfer, or may not be capable of finding music files contained in logical folders.
Try a simple trick first, see if the USB port has a sufficient 5v power supply. Just plug in the Sansa, and check its display. Do you see the battery indicator flashing, indicating charging? If so, a simple modification to the USB cable will allow you to use the USB port for charging while listening via the Sansa’s headphone jack, connected to the AUX connection.
The key here is to use the %v supply to power the Sansa, while having no data on the two center pins, so the device will run manually. The original Fuze has a nige legible display, and offers rapid navigation via its scroll wheel. Simply place a small piece of tape (clear Scotch type tape is best) over the two center pins of the USB connector, then plug in. The Fuze will think it’s connected to a USB charger, and will operate normally.
Equally simple is to purchase a basic cigar lighter socket USB car charger, and connect the Fuze cable.
I like using the USB port, as it has a smaller likelihood of ground-loop noise, or alternator whine, since the power supply and audio output are grounded at the same head unit.