Here is one of my best tools when trying to
track down electrical gremlins in race cars,
and I made it for less than the cost of gas
driving to my local electrical part store.
You will need to pick up, at least, a red/green Bicolor-LED and a couple 1200 ohm resistors. I sneaked into my wife's sewing cabinet and borrowed a large needle. Next, I scrounged through the garage and found 2 alligator clips, a piece of two conductor 22 awg wire (most any light wire will work), shrink tube or electrical tape and a clicker style ball point pen. Source your bits and pieces as you dare and then follow the instructions below to make your own test probe.
First, solder a resistor to the end of one wire and use a short piece of shrink tube to cover the body of the resistor, solder joint and back to overlap the wire's insulation. Repeat the process for the second resistor and wire.
Next, twist the exposed ends of the two resistors together and solder them to the short leg of the LED. On the long leg of the LED, solder a short wire connecting the LED to the needle (this wire should be just the right length so the needle will stick out one end of the pen and the LED out the other end of the pen). Now, use a piece of shrink tube (or tape) to cover the solder joint of the needle back to the LED bulb.
Disassemble your donor pen, dump the guts, and drill a small hole in the pen for the wires to exit the barrel. Next, feed the free end of the wires into the center of the pen and out through the hole you just drilled. Carefully feed the needle and LED into the pen as you pull the free end of the wires and screw the pen back together for the final fit. I selected a pen that had a rubber finger grip that I slid over the wire exiting the pen to keep it secure. Lastly, put the alligator clips on the free ends of the wires and a dab of super glue on the pen at the needle to hold it secure.
Connect the alligator clips to the battery and you are ready to test. As you are probing, when the LED is green, you have a positive (or hot) wire. If the LED is red then you have a negative (or ground) connection. If the LED is not illuminated, then you have an "open short."