Well first check your battery voltage readings with the car off and on, then I suggest doing as 86560SEL suggested and replace the Voltage Regulator. 2 screws and it's in your hand, literally. The brushes wear out over time and can cause issues like this. Throw the one you remove into the trunk and forget about it. It's one of those "in case" items you should have in the trunk in case you are stuck in the middle of nowhere 5 years from now and your VR is toast...Might just get you home. I got an VR, OVP, Fuel pump relay and all fluids in the trunk of both of my cars. You never know!

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2016 Monsoon Gray Audi Allroad - 21k
2008 Black Mercedes E350 4Matic Sport - 131k
2014 Jeep Wranger Unlimited Sahara - 62k
2003 Gray Mercedes ML350 - 122k