Hi Mike,
It is probably either a bulb gone or going so that the oxidisation in the
bulb is reducing the light
output. On mine ( Picasso HDi 2001 ) it's dead easy, the rim of the oval
display just
pulls out and the display unit is held in with a couple of screws. Don't
pull the unit out too forcibly as
it's got wires connected to it but you should be able to get to the back of
it. Getting the bulb holders out may require a pair of fine pliers
or a child with small fingers if you have one handy.

They are completely useless in sunlight if you have sunglasses on, and even
worse if you like to
drive in daylight with the headlights on.
TonyB