The only thing I could think of with the sat nav is to find a crashed one of the same import year and do the switcharoo, satnav wont work but aircon should.. Mate had a Honda with the same deal.
However for the stereo you could remove it and try and figure out how many channels it has. It may have five, two being front speakers, two being rear speakers and the fifth being the low frequency mono sub channel. In this case replacing the head unit in hope of powering the sub would mean buying an external amplifier to power it etc.
If your lucky though, you may find the sub is already powered by a little external factory installed amplifier. This could result in buying a replacement head unit and just hooking up the front and rear channels as normal, followed by hooking up the line to said amplifier.