It is carried by Thompson, whose corpse is on the scaffolding on 17th Street in the Pennsylvania Avenue district. To get to his corpse, climb the fallen lamp post to jump up to the first level of the scaffolding.
The explosives have been placed along Pennsylvania Ave as instructed. The electrical switch is located on the first level of the scaffolding on PA avenue. Simply flip the switch, take cover, and get ready for one hell of a show. I'm going to stick around a little longer to see if I can pick off a couple more mutants.