The 300SDL can lock/unlock the car at 3 locations, driver door, passenger door and the trunk.

IIRC, the passenger door has an interlock at the ignition switch which can override the lock/unlock function. If it goes wrong then the passenger door function may not work.

One thing I know for sure is that you have no vacuum issues as you can lock/unlock the whole car from the trunk. It is more likely electrical.

I need more info to help you. Can you describe what exactly is wrong?

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Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.