I have to say that this site is so full of helpful information which lead me to a quickie repair job on a blanked out CRT screen. The function buttons were working, but no display could be seen. The first time this happened I pulled the dash apart, checked the wiring connections and reinstalled everything. It started working again, much to my surprise. It has been fine for the past few weeks, but I let the car sit for a week or so and when I jumped back in to go for a drive.... blank CRT again!
I checked the DIY section on CRT and the RAP relays were mentioned as a possible problem, assuming the 25 amp fuse was still intact. Fuse was fine, so I pulled the panel and swapped the two RAP relays from side to side and the display has come back up again! Yay! I assume that the relays are some kind of generic GM/aftermarket part, so I better go line up a couple of them for the future.
What could have been grim news, turned out to be a simple fix. Thanks again!