Check the illumination bulbs. When it happened on my ACC display, a
bulb just needed wiggling. On others I've seen, the bulbs can go
silvered and need replacing.

Cheers,

Colin.

It's most probably due to the light dependant resistor in the analogue
clock or SCC trip computer. It can be quite difficult to replace, so
you might be better off buying a second hand unit. Plenty show up on
eBay.

There are three lamps in  the electronic display unit (EDU).  Its
possible that one or more are burned out but usually if one is gone the
lighting is uneven.  The intensity of the info display is controlled by
a phototransistor circuit in the clock/trip computer.  If the
clock/trip computer display is getting faint as well then it is most
likely a defective phototransitor.  It is a common complaint.  I
replaced mine with the only phototransitor Radio Shack stocks.  It does
not have quite the same specs as the original so the display is a
little brighter than it should be but it still dims at night.  Details
on its replacement can be found with a search of the 9000 forum at
www.saabnet.com.  One of the posts is in my name (R. Frist).   Good
luck.