Any Saab enthusiast ever seen this?

1995 Saab 900S

The dash lights only light up intermittently and fade out shortly after
coming on ...
The mileage is also always dark - here's the clincher - if I unplug the
SID the mileage
lights up and dash lights work.

Also - when the SID is plugged in and I try to view the miles until
empty, it counts in the
wrong direction ... i.e. the number of miles until empty increases as I
drive (happens quickly)

any advice / help would be greatly appreciated - thanks!