In general the X26 connector seems to be under the fuses ("lower part of the fuse box") on W124's. The left one is X26, the right one X24. X26 is "Interior/engine connector 12-pole". Remove the 6 screws that hold the upper fuse box cover on, remove the cover, and you can lift the fuse rack up/out - there is slack in the wires.
Hope this helps.
Oops - It seems that X26 could be the rear connector and X24 the front one on some models. Anyway it is under the fuse rack.