In the Junkyard level, there is a big fat pizza man hanging. Blow him up with a couple of gas cans. After he is blown up, the debris will leave a ramp to jump onto the trash compactor. Ride it all the way up and drive on the the little platform. Turn around and face the trash compactor and there is a black box to the left. Jump off of the little platform to get it. It will unlock a new level for two player modes.