Some pre-Burning Crusade schematics require Fused Wiring to create. This can be obtained in three ways. The easiest way is to craft them directly through the Fused Wiring Schematic, but this requires 275 skill to learn. It can be bought in Shattrath City and Winterspring.
Another way is to produce target dummies and loot them as they expire or die. The higher tier dummies (Advanced and Masterwork) have a higher chance to drop fused wiring.
It can also be found on expired / dead Compact Harvest Reapers, but those cost more ingredients then advanced target dummies to create, and are as such not a good schematic if your only goal is to get wiring.
Another way of collecting the wiring is by farming. The two shredders in the venture co camp in Stranglethorn Vale have a ~7% chance to drop fused wiring.
Gnomeregan is the best place to farm wiring, as many of the mechanical units in the instance have a chance (albeit a small one) to drop it.
A robot called Clunk in the Searing Gorge has a 11% chance of dropping wiring, but he’s a level 48 elite and you’ll have to kill a lot of stuff to get to him. He probably doesn’t respawn too fast either. Not a very good way to get your wiring.