The lights are on
Okay first go to the multiplayer server and select TDM (Team Death Match) and wait for "Nuke town". Then if all of the mannequins heads are knocked off with in the 1st 30 seconds that the map starts a light will flash and "Sympathy for the Devil" will play by the rolling stones while you kill the other team.
It does not matter how you knock the heads off you can knife them, shoot them, place C4 on them or toss a grenade but all the heads must be knocked off before the 1st 30 seconds so work fast. Also it doesnt matter if the other team helps and the recomended team size for this is between 10 and 12 ppl.
Note: each time you play Nuke Town the mannequins position will move so its not helpful to memorize where they are during one round because they wont necisarily be there the next round.