So For broke purposes I am only replacing the subwoofer and the adding a dedicated amp for it but I wanna keep the stock amp for the speakers till I render them useless. I read zeroflux post and I'm stuck on one part when my line converter arrives tomorrow would I use the 4 wires from the harness or would I only would have to use 2 ? The line converter would it go right before the pins of the amp or would it go after the pins of the head unit ? Please help a brother out ! Btw it's a 2015 gsr sss

 

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If you're using a line converter, you'd intercept the amp output to the Sub, at the sub.
Otherwise, a line level signal should be going into the amp under the driver's seat.

If you're using a single Voice Coil sub or a mono amp you can grab just one channel.

The factory HU simply puts out a Left and Right signal and the amp handles the fading through CAN BUS as well as the crossover work internally.

The inputs from the factory HU to the amp, which you could tap to grab a line output, are pictured here:

 

What line converter are you using?

That will determine where in the system to pull signal from.