My Navman has two slots, one for the C-Map and one for an SD Card. You download all of the marks onto the SD Card and use the PC-Planner software & card reader to open on a PC, from there you can at worst print out a copy of all your marks and re-type them in.
(For some reason you have to use their card reader, it won't load with any other reader, that I am aware of).
It might be worth asking the guys at Trymax marine if they have a solution for the backlight failure?
Not sure how easy it will be to get a hold of the c-map planner software and card reader?