I have seen a lot of MBs that wouldn't open the trunk unless you turned the key all of the way CCW and hold the key at that 45 degree position while you push in the lock.

Some times the vacuum diaphram pushes the lock mechanism back to the lock position even if the key is horizontal.

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Ron Schroeder
'85 300 Turbo Diesel 2 tank WVO
'83 300 Turbo Diesel 2 tank WVO
Some former WVO vehicles since ~1980:
'83 Mercedes 240D
'80 Audi 4000D
'83 ISUZU Pup
'70 SAAB 99 with Kubota diesel
'76 Honda Civic with Kubota diesel
'86 Golf
Several diesel generators
All with 2 tank WVO conversion
LI NY