OK, so here goes.
Remove the plastic sill covers, there is screw at each end (posi drive) then a sharp (but carefull) upwards tug.
The reamove the centre console (the bit in front & beind the handbrake) by removing the nuts (10mm) under the rubber cup holder liners & 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) the run the L/H speaker pickup wires (white/white & black) & the earth wires (black) right across the car to the L/H side.
T into the yellow (+) with the white wire from the sub, and into the yellow/black (-) with the white & black wire from the sub.
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

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