i went a slightly easier way and bought a used taillight harness off of ebay, and just used the 2 sockets that have all three wires (major, minor, and ground) for the third light. i took off the 2 wire socket and replace it with the 3 wire socket, then just spliced the wires onto the neighboring original 3 wire socket. its a parallel circuit, not a circuit in series, so the brightness of all the lights wont diminish.
because i wanted to keep the original harness un-molested (i did this way before i did the export light upgrade) i didnt cut off the 2 wire socket, i just left it hanging there, but i took off the bulb and used electrical tape to cover up where the bulb should be.
what you can do tho, if you dont want to buy another harness or sockets, is get a piece of wire and stick put it in the socket so the 2 wire socket now becomes a 3 wire socket (the missing wire is the major wire, the rubber protector comes off easy enough and then you can use a paperclip or a needle to make the hole for the extra wire). the tricky part would be to make sure its always making contact with the bulb. i *THINK* you can just put it in there and use the bulb to hold the wire in, but i can remember exactly how everything is positioned inside the socket. if you can get it to hold in there, then you would just need to splice in that wire to the neighboring socket and you should be all said and done. at the time i didnt know i could get away with doing this, so i bought the harness.