Here is a diagram showing how I think the powered sub should be wired:

 

We got a Kenwood powered Sub for my daughters Subaru and that's how the install shop hooked it up...

 

Thanks for the reply ..

 

High level = amplified speaker wires (even if from the OEM head unit)
Low level = RCA cables (non amplified signal wire)

Yes, your diagram is correct.

 

while yes you obviously do not want to tap into the amplified wires and take signal from the head unit before the amp you still should check to be sure with taking the signal from the rear channels.

1) on fader/balance alot of us will obviously have the tuning towards the front= less output on the rears= less output for your sub= need for more gain= distortion.

2) alot of cars today have a volume reduction,filter etc right from factory for the rears = agian much weaker signal for the sub.

I personally would always take signal from the fronts for a powered sub not rears.

 

Thanks everyone for your input ... I will use the front speaker level / non-amplified (head unit to amp) signal for the powered sub.