BMW E46 Alternator DIY (voltage regulator)
Whether you've replaced your alternator or not, do yourselves a huge favor and check for a loose battery ground cable in the trunk of the car. Look at where the ground cable is connected to the body of the car, under the trunk mat.
If it's loose, remove the fastener and clean the carbon from the area with a battery terminal cleaning product.
A loose connection on that particular cable is common in the E46.
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