I'm not sure, but maybe the dealer can sell you a tumbler keyed to your orignal key which works in the ignition, and trunk.
You never said if the key works in the passenger side, but I am assuming only the drivers door lock was changed.
Check with your local dealership, while you are at it, you can order some fresh keys form the factory too.
Or perhaps the tumbler from the passenger side can be put on the drivers side. that way the other key will work in that door while the main key works in the 3 you want.
'92 300CE - Sold
2004 C240 - C7 Wheels
2015 ML350 - P1, Pano, Ash poplar wood, Sport Dinamica interior, Running Boards Keyless go.