The battery light on my 240D has been on for the last several days, though it's been quite dim. I confirmed that the light hadn't been lying to me when I found out my battery was totally drained and I couldn't start the car. I installed a new battery from one of my other cars and the car runs fine but the light is still on so I stopped driving it. Does it sound like all I need is to replace the brushes in my alternator or do I need a new alternator? If so which one (55 or 65 amp)? If not, how do I replace the brushes?

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2004 VW Jetta TDI (manual)

Past MB's: '96 E300D, '83 240D, '82 300D, '87 300D, '87 420SEL