So my old amplifier died (from the wires going to the subs shorting out) and I'm replacing it with a Kenwood KAC-9152D (head unit is a Pioneer DEH-8500MP). Before I was using a converter that was spliced into the rear speakers to give the amp the signal, but I decided to run RCA cables this time (connected to the RCA jacks from the head unit labelled Non-Fading / Subwoofer Output).

The issue is that if the amp is not on, sound comes out of the in car speakers (as it should), but once the amp turns on (when the car is on) no sound comes out of the speakers in the car anymore. I don't know if the head unit thinks it shouldn't power the speakers if it can sense something connected to the RCA outputs, or if it's a grounding issue (I didn't check the ground cable connected to the amp (left over from the old installation), maybe it's loose?), or something else entirely.

Anyone have any ideas?

I don't know if this means anything or not but when I pull the fuse out of the power cable (+) the amp still lights up when the car is on - I'm guessing it's gettng power from the remote accessory cable. If that's right, then it means the ground connection should be fine, right?