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The instructions are found here: http://dsl.torque.net/images/Markerflash.pdf
Here is how simple it is on a Spider:
Go get two of those crimp on "add a wire to an existing wire" splices
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Open your hood and locate the star like multi-ground wire posts toward the front of either fender. Figure out which of the connected ground wires is for the side marker. No rocket science here, just turn on the parking lights and unplug one wire at a time until the marker on that side goes out; that would be it's ground wire!
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Now locate right there as well the blue/black wire (harness side of connector) or light blue wire (turn signal side of connector) for your existing turn signal. Using one of the crimp on "add a wire" splices connect the ground wire for the marker to the blue turn signal wire. Tape it all up good and go repeat the process on the other side.
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Your side makers will now function just like your turn signals: they will flash as turn signals with lights on or off and still act as a lit side marker when parking or head lamps are on. Go do it now!