Modern technology has come up with a number of items that make working on your car quicker, easier and more efficient. One of these - shrinkable tubing for insulating wire splices - is a pickup from electronics and is available in many electronics stores. You can even get in colors, to match the insulation. Simply slip it over the electrical splice, apply heat and it shrinks, forming a tight-fitting insulation.
Don't buy a lot: A four-foot length (the smallest length sold in most electronics stores) will insulate many splices. Also, make sure you buy tubing with an inside diameter large enough to slip over the wire you're splicing. If you don't trust your memory, cut off a short piece of the wire and take it to the store with you.
Shrink tubing is pretty cheap. A four-foot length of 1/8-inch tubing, which handles 18-gauge wire (a common size in cars), should run less than $1.50.
Cut off a piece of tubing long enough to cover the bare wire plus at least half an inch on either side. Be sure to put the precut length of tubing over one of the wires before you make the splice.
You can apply heat with a soldering iron (which you just finished using to solder the spliced wires together) or a match.
Don't let the soldering iron rest on the tubing, just let the hot tip almost touch it. Make sure you apply heat to the entire length of the tubing. Keep applying heat until the tubing has shrunken tightly around the wire. Withdraw the iron and you're finished.
Followed the same principle with a match: Pass the flame underneath the tubing, making relatively quick strokes back and forth across the length. Holding the flame in one place will quickly melt the tubing. Don't use a match in a crowded or enclosed area where there's a chance of catching other components on fire.