Its too easy
Just buy these two reverse connectors & one forward connector shown here:
Let Dave know what you're doing and he'll make sure you get the right parts.
For the end that will be plugged into the subwoofer under the driver's seat take one forward and one reverse connector. Then twist the matching colors of the two connectors together. You now have all 14 wires from the two connectors paired up.
Figure out how long a wire you need to get to your location for speaker #2. Next grab some similar gauge wire. Cut 14 equal length pieces for jumpers, I think mine were around 3'. Strip 1/2 inch of each end.
Now pick a color to start with, say orange. Twist the jumper onto the orange wires for the two connectors that will go under the driver's seat. Solder together. Connect the other end to the orange wire of the reverse connector that will go to speaker #2. Solder together.
Rinse & Repeat until all the matching colors have been soldered together. Double check your work. Tape up (or you could have used heat shrink like me).
Unplug the original connector from speaker under the drivers seat. Plug it into the forward harness connector off the Y cable. Plug the reverse harness connector into the subwoofer under the driver's seat.
Route your cable to the speaker #2 location. Probably want to go under the console. I am going to swap out my carpet so that's all I did at this point, and you can't really see it any way.
For the bracket that holds speaker #2 under the rear of the passenger seat, just put the portion of the bracket that fits under the seat frame in a vice, and bend it the opposite direction. Then lay it out on the floor w/ the speaker, and drill new holes in the bracket to hold the speaker where you want it. Alternately slap some velcro on the back off speaker #2.
Now plug the reverse connector into speaker #2 and enjoy.
When I get time to swap my carpet I'll snap some pics, but there's nothing to it really.