I had a under seat sub in my 05 Wrx (panasonic EAB1689A) that I am trying to connect to my 2006 G35,

There is 6 wire on the connector on for the power on for the ground and then 2 for the left rear speaker and 2 for the right rear speaker.

I never pull out my factory radio so I don't know if there's extra connection for those wires or not, And I know that you can't hook up two speaker to one output because of it's impedance.

thanks for helping me out.

There are not extra plugs on the back of the stock radio.

I dont think you need to hook up more than just the front OR rear

as long as it gets a signal it should work fine.

the sub shouldnt lower the impedance at all
they are made to run in series as to not lower your impedance