Oddly enough this happened to me yesterday on a trip. Was the battery low when this happened?

The connector is a Mini-USB series B connector which has five pins. What I imagine is happening (maybe) is that normally only Pins 1 and Pins 4(and 5, connected to Pin 4) would have voltage, as these babies would be the power in and out. Pin 2 and pin3 would be data, and if any voltage detected on these pins, the device would go into usb mode. So maybe it's dust or some kind of crossover? Is the charger you have definitely transmitting 5V? Your car electrics ok? I can't see your car could be the issue. Although try another car I guess. Then ditch the old one if needed.

What I did was to turn it off and leave it to charge for a half hour, then plug it in and pray.

Mind you that was after repeatedly plugging the charger in and out of the lighter socket several times.

Maybe a blast of compressed air over the mini-usb end of the cable might clear dust out of it.

Saying that I can't say that'll fix it. Would it be an issue with the Navman end of the usb plug?

The navman should be detected as a hard drive, however the Smartst2008 should detect the device through it's API and it can do it's magic. If this is not the case, have you tried uninstalling and re-installing the original software supplied with the unit? Could be the drivers maybe.