Doors failing to lock?

[avlis]

Both drivers side doors on our R350 fail to power lock. They will unlock with the keyfob but the pins wont go down to lock. The rear door makes a faint buzz while pressing the lock/unlock buttons. Anyone have a problem like this fixed? I was thinking of just replacing the rear lock motor because it's making a buzzing sounds unlike the others. But would a failed lock on the rear door affect the front door?

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[avlis]

Update: I replaced the driver side rear door lock using instructions I printed from Startek ($18) and got the part online ($255). I chose to do this one first since it was buzzing when trying to actuate. So that door is fixed. The front door lock is still non-functioning so I ordered that one too ($255 again). Must be a coincidence that both locks on the left side failed around the same time. I'd hate to know what a dealer would charge for this fix. I'm guessing $1200+.