rheostat is still ok, is it a computer fault and can a piggyback management system sort it ?

Is that the one with the 'night panel' thing that turns all the cabin lights except the speedo out? If so, maybe there is a fault with the switch.

l had a 900 which developed a fault in the electrics, the front panel (dashboard) has a flexible sheet within it & this had 'cracked'. l removed it & got some conductive paint (electrodag) & joined the break back up.( l was lucky to have access to small quantities of electrodag - its not cheap)
This may not be ur problem but worth thinking about. The Haynes manual had a full wiring diagram l recall.
It was funny actually as we'd bought it second hand & taken it out in the daytime & the one thing we had not thought of were the lights

you probably have a socket or a board failure. Seems that something is failing under a load

The cluster lights r the only lights in ur SAAB that r dim-able. On ur dash panel on the lower left u have a dimmer switch. Most likely one of two things has happened. Either just due to heating & cooling of the switch over the years it is trying it is best to die, or, the little fork that holds the setting has popped off its knob. If when u roll the wheel on the dimmer there is NO resistance at all, it has popped off. If there is resistance felt then it is dying & u will need to replace it. It is not expensive & u can install it yourself, it just snaps into the dash. Any questions just drop me a note. Good Luck