[ISpringer]

Do the idiot lights still come on when you turn the key? If not then likely a fuse. On the 90 there is a 5 volt regulator in the cluster that can go out. At least on mine there was.

EDIT
Nope, it was the wifes Volvo I did that on.

 

[ISpringer]

Are the gauges illuminated when the headlights are on? If not it's definitely a fuse. I'd give them a good checking first.

There are a pair of 15 amp fuses and a 5 amp that power the instrument cluster. I can't seem to find which one exactly is used for those two items though.

 

[ISpringer]

I hope you have a tilt column, as it seems like you need to pull the instrument cluster. The tach and speedo are a PITA, but it comes out fairly easily. I'd need to tear mine apart again to see exactly how it's built, but it could be an internal connection within the cluster.