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I have recently purchased a 1999 996. The original manual was missing, but I was able to get another one. The manual doesn't discuss one of the buttons. It is the bottom rocker on the left side of the stereo (see pic), just below the wing extension button.
On a glass top targa, that botton retracts the top. I you case, PO must have installed it for use as a 'garage door opener'; the mod is discussed under that topic. Do a search.