I am always looking for materials for my models. One of the things I have always had a problem with is scale wiring. While some aftermarket wire is close to scale, it is very expensive. I hate to spend two dollars a foot for small gauge wire.
Looking around I found several good sources for small scale wire. There is a seller on the web that goes under the name Inventex2012. He offers ten feet of ten different colors of 30 gauge wire that goes for $5.80. Less than 6 cents a foot. This is the same stuff most aftermarket companies use.
This wire is .5 mm outside diameter. In 1/24 scale that is a half an inch. While it is probably a little large for plug wire in scale, I think it looks good.
If you want to go smaller, there are two common sizes of beading wire. I found rolls of .3 mm and .4 mm beading wire in a variety of colors. These rolls come in a set of ten rolls each having ten meters of wire. These cost about 8 dollars for 100 meters of colored wire. the .3 mm wire is about 3/8 of a scale inch and the .4 mm is just under a half inch in 1/24 scale. These are coated wire and are shiny. I couldn't seem to break the shine with sanding without going through the coating but a coat of clear dull paint made them look better.