Yep, Right up in the second (top) level of the control room, accessed via a lader in the far corner of the room, after you complete the objective in said room, there is a narrow lift against the wall that you need to take down to the very bottom of the tower.
This is the very last objective, once you make it outside via the sewers, you pick up the detonator from the ground in front of the dying engineer and then detonate the explosives at your leisure.
Thanks for the help...It really helped me...Thanks a lot.
I owe you one.