I still wonder about a ribbon issue causing a total loss of the display. As we know, there are about 60 separate circuit connections from the motherboard, via the ribbon, to the display. Has anyone ever had them all disconnect at once? usually the display starts to "break down" gradually, which is why I suspected the backlighting filaments.Is the new ribbon cable going to fix the dead pixel issue (when i put it on the SID)
Maybe one of those numerous contacts is the earth return, and its the one (or one the ones) that disconnected. That might explain total blackout. A shot in the dark perhaps, but the following might cure that.....
While you're waiting for the new ribbon you could take the SID apart and try the temporary fix, which is to try and improve the ribbon connections by (a) increasing pressure by inserting some foam between it and the casing ( a bit of a faff, and not reliable) or (b) running the tip of a clothes iron (soldering iron is too hot unless you have the special 3 cm wide head) along the connections to melt the conductive adhesive or whatever it is and hope they all re-connect.
The latter method has worked for me on several 9-5 SIDs. I believe the car still operates without the SID and if I'm right, you have nothing to lose. Otherwise, if the ribbon fix doesn't work, get a used SID, preferably one with a photo showing it working.