Unplug the switch and check the wires through the plug with an ohm-meter. If you don't have one, you can pick up a cheap one at any parts store or even Wal Mart for around $5. Make sure it has batteries or the ohms readings won't work.
Set it on ohms and check each wire to chassis ground. They'll all give you a reading because you're reading through the bulb filaments. However, the lowest reading (nearest to zero ohms) will be your short circuit.
Be aware, though, if you're not familiar with analog ohm meters, that the needle needs to be "zeroed" by holding the two probe tips together and setting it with the knob. Also, the lowest reading in ohms will be the reading that moves the needle the farthest to the right (you probably noticed this when you zeroed the needle).
Once you determine which wire is shorted, use John's color info above to tell you where the wire is going.
BTW, give me the specs of your car and I'll tell you how to get the mileage above 20mpg. I want those free cookies.
Mine gets 21-22 mpg on the highway with 3.55 gears and it ain't slow.
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'68 El Camino
[This message has been edited by cjlandry (edited 10-02-2002).]