My first guess would be the alternator...if your headlight intensity changes with engine speed (other accessories off), it's not doing its job.

More than likely, it's the brushes or the regulator, both replaceable components on certain alternator models.

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car