Suddenly, key does not lock or unlock rear or front passenger side door, and automatic release for trunk and fuel door do not work...anybody?

You might want to check fuse #2, 10amp.

Does the remote lock/unlock the doors?
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Keeping it running is better than buying new

Thanks for response...never had remote--just valet key...but it always worked as it should before yesterday. Checked and replaced #2 fuse but no change. Getting low on gas too...sheesh. Just a FYI...earlier same day installed a Premium Sound (815) CD/cassette/radio from a 96 960. First indication of a problem was next morning when I opened door with key the alarm sounded briefly...restored old radio just to see if it would affect situation but it didn't. Any direction is greatly appreciated.

You can open the filler door by removing the right side trunk paneling, at least pulling it away from the fender just enough to get to the release mechanism. No need to break anything.
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Keeping it running is better than buying new

Found the fuel door release; thanks Klaus. It was advised to swap the Central Locking relay located somewhere under drivers sude knee bolster. Anybody know JUST where and/or what the relay looks like. Looked but see nothing marked as such. Thanks!

The 2000 is a problem. Some have a relay, some have a central locking module. It is also possible that a door lock, not just the driver's door, is broken.

Have you tried the lock/unlock switch on the driver's armrest? I would guess you did and it didn't work.

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Keeping it running is better than buying new

I've tried evertthinv you noted. How to differentiate models where it utilizes Central Locking system vs. relay? Anyone else had any expere*nces repairing ttbis issue?