I've got the same jaycar amp in my older nissan and the whine is really bad at lower volumes, maybe the amp just needs a filter no matter how it's installed.
I've got my rca's and power run on opposite sides of the car, completely seperated whereever possible, got good quality rca cables, upgraded the ground connection between the battery and chassis to a 0 gauge cable, and have tried relocating the ground from the amp a couple of times to get rid of the whine with no luck.
It's a great amp otherwise, I've got it with some jaycar kevlar response splits which are okay but would probably sound a lot better with some decent sound deadening.