Heyo!
I've been looking around at how to get a second Haywire Blaster out of the Scourge box set, and I think I found an easy way. I'm new to Dark Eldar and the model range, and I'm sure there are some details that don't match, but I think this will qualify as close enough.
The components you will need are an exacto knife (really any modeling/cutting blade will do), glue, a Heatlance and blades from the Dark Eldar Warriors box set.
The first thing I did was cut the tip from the heat lance...but in hindsight, I think all of the cutting can be skipped in this step.
I then shaved some of the tip off, just so it more closely matched the actual Haywire Blaster.
Split the Warrior Blade into 2 equal parts, you won't need that middle bit.
Then, just glue it all back together. If you skipped the cutting in the first step all you need to do is glue the blades on.
And a shot comparison with an actual Haywire Blaster.
My apologies for the blurry pics, all I have is my iPhone.