OK, so here goes.
Remove the plastic sill covers, there is screw at each end (posi drive) then a sharp (but carefull) upwards tug.
Then reamove the centre console (the bit in front & beind the handbrake) by removing the nuts (10mm) under the rubber cup holder liners, one 10mm nut just in front of the hand brake & the two screws (torx head) at the leading edge (carefull to unplug the cigarette lighter & the Blue & Me plug)
Loosen the rear bolts on the seat runners & remove the front bolts completly (allen head screws).
Feed the R/H speaker pickup wires (grey/ggrey & black out into the right hand loom run (I have left these disconncted at the moment as I cannot find what wires are the R/H speaker wires, but the bass is normally non stereo anyway & the manual says this is OK to do)
Then you should be able to pass the wires through the hole in the carpet where the drivers seat wiring goes & along the cross member.
Connect the two red wires into the wire the goes into the cigarette lighter (grey/red wire). I popped the metal female spade from the cigarette lighter plug & soldered to the metal part then reasembled the metal part into the plastic block.
Now run the L/H speaker pickup wires (white/white & black) & the earth wires (black) right across the car to the L/H side, this is why you need to be able to lift the front edges of the seats as it makes getting the wires under really easy.
T into the yellow wire (rear left speaker +) with the white wire from the sub, and into the yellow/black wire (rear left speaker -) with the white & black wire from the sub. I used soldered joints with heatshrink tubing ever, but Scotchloks cam also be used.
Finally connect into the earthing point at the front of the door aperture that will be clearly visable (10mm nut), I used a 6mm crimp on connector.
Then locat the sub under the drivers seat with some hook velcro pads to stop it sliding aroung & plug in the check the function.
If it all works it's time to but your 500 back together again

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Be very carefull that the wires are running behind the seat runner pickups & are not being pinched as you tighten the seats up again.