Willet's Point in Queens might be just your bag:
http://en.wikipedia.org/wiki/Willets_Point,_Queens
"Another euphemism for blight" is what our mayor calls it. If automobiles in various stages of decomposition are your thing, that might be a fun place to poke around in. Catch it before the gentrifiers get a hold of it. It's basically a giant chop shop -- I'd be less worried about people out to steal your camera, perhaps, than people simply wary of snoopy photographers.
After a day of shooting you could treat yourself to a good Chinese meal just a stop or two away in Flushing, or choose from a fantastic variety of "ethnic" restaurants a few stops the other direction in Jackson Heights.
Most of the "decay" in New York is a case of the fish rotting from the blind head -- it's more an internal institutional corruption that's the problem. As far as ordinary people go, I see less decay than efflorescence.
Oh, if you happen to visit Central Park, and you're a "Law and Order" fan -- don't be disappointed if you don't discover a corpse in the bushes. Seldom happens. :ugh: