Have you carefully, methodically, and without skipping steps, gone through every single one of the diagnostic steps outlined here, here, here, here, and here?

Have you re-done the wiring from scratch, using only your own wires directly to the battery instead of using the car's wiring harness?

Have you removed all components from their mounting locations so that they are floating free and not touching the vehicle frame?

Have you tried increasing the wire gauge on all of your ground wire runs?

Have you tried grounding all components to the same point (preferably the battery neg terminal) using heavier-gauge wire?

Have you double, triple, and quadruple-checked each possible wiring connection at all places (your docking harness, the amp, the battery, all connections in between)?

Have you checked to make sure your battery isn't simply low and not charging well?

Have you tried a different set of RCA audio cables?