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The instrument cluster does control the illumination as well as the interior lighting, so if the cluster went down then you would also lose the illumination on the radio and a/c control head.
If you aren't having trouble with any other systems when this happens then I agree that it's most likely an instrument cluster issue. It's possible that could be losing power or ground though.
The first thing I would do is take a close look at the connector terminals to make sure there is no sign of spreading, corrosion, etc. Try and clean the male and female terminals with connector cleaner and see if that has an affect on the issue at all.
If nothing changes then most likely you have a cluster problem. The best thing to do would be to verify it's powers and grounds are ok at the time it's not working, and if they are then you know the problem is internal.