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If the stereo light is actually good, the most common problem is alignment. The tuning meter has a relationship to the ratio detector- if the ratio detector is out of alignment, the correct setting of the meter may not be at the peak. If the unit has this problem, it may not be able to detect the stereo subcarrier, thus the stereo lamp will not light.
So alignment is the first thing to check. For this you need actual test equipment- an FM Stereo generator for starters.