Well, I've tried a whole bunch of things...
- Hooked up mp3 player to amp. No hiss at all. Blissful silence. I want to believe that this means the amp and the new wiring to the speakers is not the culprit.
- Removed the male-male rca extensions and swapped out the female rcas on the harness for male ones. Plugged harness directly into amp and the hiss returns.
- Grounded amp at battery ground post on right bumper, no difference and hiss is still there.
The hiss is noticeable on front doors, but is particularly noticeable on the rears.
This is rather dejecting. What are my options now?