1988 mercedes 260E stuck fuel flap cap, how to use manual release in trunk?

For some reason, the flap on he fuel cap won’t release. (I think it has something to do with a malfunctioning vacuum system that operates the door locks.) There is a manual release in the trunk (it looks like a circular knob on the ceiling inside the trunk near the fuel line) but I can’t seem to figure out how to use it. This is my wife’s car. She adores it. Runs like a top. I need to fix this for her. Thanks.

You should just pull the trunk release to pop the gas lid. If that doesn’t work, I’ll take a closer look at my 300E when I see it again on Monday and post back.

I’m wondering if something else is causing the fuel door not to open. It should swing open when you push in on the end nearest the hinge. The hinge is the pivot point. You should still have to push it open when you pull the cable inside the trunk.

What are the door locks doing when you unlock/lock the driver’s door or trunk?