Amp install troubles 2013 prius

I am trying to install an amp and sub in my 2013 prius.
I purchased an "add a sub" wiring harness off of ebay so I could retain the factory head unit.
It's a nice little harness that has RCA and Rem wires alread hooked into it.
Well I have everything hooked up good.
The green light on the amp comes on for start up but as soon as music is turned on the amp goes into protect mode.
I had the ground attached to where the battery ground goes.
And even put it on the battery itself.
Same issue.
Even went up to a larger guage wiring kit and same problem.
I don't know what else to do.