If you get the vacuum system working, the trunk may open with the knob without a key. As I recall, there are two ways of locking the trunk. One way locks it so that it will not open when the vacuum door locks are opened. The other way, unlocks the trunk when the vacuum door locks are unlocked. If the latter is activated, all you need to do is push in the button and it opens. (At least this is how I think it works on the 77 300d I have).
Alternatively, if you can isolate the vacuum line to the trunk lock and pull a vacuum with a Mity Vac while praying that the lock is in the latter configuration, you will be ok.

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13