Try removing fuse 9 in the rear fusebox for about 5 to 10 seconds, then reinstall fuse and try opening and closing the trunk a few times. If it works, I can explain whats going on.

Another idea is to look in the owners manual, there ia I believe a small lever in the underside of the trunk which can disable the pull-down system for the trunk, possibly it got shut off by accident.

Gilly

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Pulling the fuse does not fix the problem, I had tried it already multiple times.
I did mess with the shut off lever on the lid originally when the problem first started (in the hope it would make it easier to close the trunk...not) , but I made sure it is as described in the manual after trying in either position several times now.
The manual seems to indicate that the latch can be moved with the lever in one position. Mine does not move in or out in any case. Could the latch be stuck?
I wis I had looked at it operate when it was still working to see which parts move and how much.

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Sab.
'07 ML320 CDI (398 ft lbs / 1400 RPM)
'02CL 500
'91 928 GT Supercharged

You are now the state that I was when my journey with trunk closing assist started. After pulling fuse 9 for 5 or 10 seconds I tricked the trunk into thinking it was closed; the handle came in and I heard a hissing noise from the close assist actuator. After 2 or 3 tries the hissing would stop but the part that pulled the latch down never moved. I removed the line from the closing actuator and kept my thumb over it to seal it and the hissing noise stopped. I searched for the part number that was on it in FastLane and ordered the part (it was around $50) and my problem was solved. All that was left to do was adjust the height so the trunk lid was flush with the quarter panels.

I hope this helps, good luck.