Could I please get that diagram?I have a 2009 Passion with a Radio 9 that came WITHOUT the OEM sub. I was lucky enough to buy the OEM sub for $50.
The plug for the sub-woofer IS under the drivers seat as Gearhead explained. It simply plugs into the sub, you mount it down with bolts and the seat and you have instant bass.
I have the pin out diagram somewhere as I wrote it down. I did car stereo installs in the early 90's and have a lot of knowledge in this area.
I'll try to find the diagram if others need it. You can then install any aftermarket sub with high level inputs, just maybe not under the seat.