We use a Raymarine ST60 which you can dim or switch off the backlight .. Check book .. Maybe that the small tube in the display is finished .. or bad contact .. These tubes can be very small sometimes only a couple of mm in diameter.
Yes, have tried everything including stripping the unit down... the screen itself has a pcb attached, cannot really get in any further. Have contacted B&G to see if they still support the Focus.... fingers crossed!