I know I am reviving a (very) old thread, but I'm currently dealing with this exact problem (1985 300D), and I thought I should also mention that the reason that a better quality part/better designed part wasnt used, is because if everything is working properly, lubed properly, etc, it will work just fine for many years.

The main problem is lack of lubrication/servicing. If someone actually took off the door panels and lubed the regulators properly once a year (hell even every five years!) they would probably still be working just fine.

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Current cars:
2000 ML55 AMG, 174k miles
2003 C240 T-Modell, 202k miles
1995 S320, 207k Miles