A headlight went out on me, and when I was unplugging the bad lamp, the black plastic thing that holds the two wires partially crumbled in my fingers. Can the black plastic piece be replaced?

 

The same thing happened with mine, when I most recently changed out the headlights. I ended up using some military grade (thanks, former army friend) quick disconnects. Kinda like these, but with no plastic boot on them (the plastic melts QUICKLY in that hot lamp housing). I spliced the wires, and slapped two of those bad boys on.

I'm not sure if you can find the plastic bits for sale anywhere, but I'd bet you have to order the entire headlight assembly.