So you have a loose wire in your car and you want to see where it goes. Well the first thing you have to find out is whether the wire is hot or cold. In other words does the wire have electricity already going to it? Or is it supposed to be plugged into a source of power? There are a couple of things that you can do to find out that are not too dangerous.
The first thing you should do is get yourself a service manual for your particular automobile. Then look in it to see if there is a picture or diagram of your wiring system. If there is one then check your engine and any other easily accessible place with wires and see if there are any missing connections.
If you are unable to see exactly where the wire is supposed to go by following it or a diagram, then you will need to find out, as safely as possible if the wire is hot. The first thing you need to do is either buy, find, or make your own tester. Usually they consist of two tester wires and a light bulb in the middle.
Now that you have your tester turn your key on and hook one end of the tester to your bumper or some other place you can safely ground it. Make sure you ground it away from your gas line because a dangerous spark could happen. Then take the other end and carefully touch it to the bare part of the wire. The light should light up but if it does not check to see if your bulb has been burned out before you touch the wire. If it does not light up and the tester is in operating order then the wire is not live.
Now that you have discovered that the wire is not dangerous you can search to see where the wire goes. If the wire goes through a hole into another compartment you will probably need a friend to look at the bunch of wires in that compartment. Now pull the wire back and forth and ask your friend to grab the wire that is moving. Now that the wire has been found on the other side search until you find the place it goes to. Remember if your car is working properly you might not want to mess with it just make sure you do not mess with anything that is working.