If the light is on with the engine running at a fast idle (>1000 RPM) then I would suspect a faulty alternator. You can check it with a voltmeter. If the alternator is charging it should read around 13.85 Volts as described above.
Note. Bosch alternators charge at a lower rate that most other makes. The others usually show as much as 14.5 Volts when charging.
There was an article in Roundel last year that gave a DIY on boosting the Bosch charge to over 14 Volts.

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