this is not an uncommon problem. the cable that unlocks the latch and the lift assist has a worn cable housing. the cable housing slips past the retaining point causing the cable to become a bit too long and not enough tension to activate the latch. you can just replace the cable and that will work for awhile. i have better results when i fabricate a washer that fits perfectly in the retaining cavity keeping the cable from slipping past that point. anyone who works on these should be able to do this in a bit over an hour.
paul

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Paul Roberts
Mercedes Centrum Inc.
Bellevue,WA.