hey, in the 33 i am guessing your amps are in the boot.
Your rcas would be running close to your battery. As a test listen for the humming while moving the rca cables.
I had to make it so my rcas went no were near the battery and or power cables.
Even if i put it 20cm away from the battery the humming would come back.
Run rcas down the right hand side (from back of car)