You could always get lead wire in the form of solder for electrical work, but usually that solder was in a very large diameter, something that would work well when heating and putting together multiple electrical connections. Now, UMM-USA has taken that one better in providing different diameters of lead wire for your every modeling need. The diameters given for review are 0.2mm, 0.5mm and 0.8mm. It's great having the 0.2mm since I model in 1/72 and 1/144 only, so that smaller diameter is great for adding things like wiring and plumbing to the inside of wheel wells, etc. In fact, the larger diameters will also be good since not all wiring and plumbing was of the same size. With the different diameters you can make the insides of "things" look more realistic.
The best thing about the lead wire is that it keeps the shape you put it in. Unlike "standard", steel wire, there is minimal bending needed to add it to places, and that makes it a lot easier to use. Plus, it will keep its shape (although with it being very malleable it is also very easy to take it "out of shape").UMM-USA has round lead wire that starts at 0.2mm and goes all the way to 1.0mm, perfect for any model in any scale.
Many thanks to UMM-USA for providing the extremely useful set of "tools" for review.