Sounds like you're guessing and just throwing parts/money at the problem without knowing what's wrong.
Measure the voltage at the battery with the car off (should be approx 12 volts) and then again with the car running (should be approx 14 volts).
If the voltage doesn't increase with the car running, you've likely got an alternator/regulator issue.
Tell us what you find.

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KenP

1996 S500 W140 Coupe