I like the idea of a harness to give a low level output, only big job then would be to run a line from the radio in the front to the sub amp in the boot. Saves splicing into the wires for the rear speaker though.
I managed to get hold of a 10" sub and like the idea of retrofitting this into the OEM sub enclosure but can't justify the cost of the enclosure so I'll have a crack at building a box out of MDF.
Its interesting with the OEM sub enclosure how much of the internal volume is actually below the boot floor. I'll have to investigate how easy that is to do in MDF as the sub will need quite a big enclosure.