Disassemble the front panel until you can solder the old blown 6V lamps out.

Put a 220 ohm resistor and an ultra bright 3mm blue LED in series in the place. Cathode coes to the bigger ground pad, which is also connected to the solder tabs at the outer ring of the assembly.

If you use another colour, you might want to use a bit higher value for the resistor to keep the current below 20mA.

This is how it looks like after the modification.

Ref: www.mods.dk