My rear driver-side door locks about 90% of the time on the first try. If it doesnt, I usually open it and then manually lock it.

What possible reasons might this be caused by, and how might I fix it?

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My guess - the lock actuator is starting to leak. You can fix it or replace it. The balance around here tips in favor of replacing it since the cost of a new one amortizes to nothing over a 20 year life. The cost of a new diaphragm amortizes to even less but you risk damaging the housing whenever you touch it and there aren't good stats on the life of a replacement diaphragm.

Dig in there with a Mighty-vac and check the hoses, fittings, etc. before buying parts.

Sixto
95 S420
91 300SE
87 300SDL
83 300SD