You say 'every part', this would be in just one corner. I know in some of the older types (other than 72 and 74, not familiar...) DC-8, L-188 and I think 707, these were places on the panel that were connected to the electrical system to power the panel lights. Quite often if the entire panel (panel, not instrument or guage...) background lights were not lit, pressing in that area would 'reconnect the panel', getting the lights back. Perhaps this.......
