Crossovers will be fine. Do you have a spare set of RCAs? Even ones that aren't shielded?
Like I said, once the RCAs are corrupted, without getting new ones, you're going to have a tough time getting rid of the noise completely, but you have to sort it out or the same thing will just keep on happening.
What spec is the amp? And what grounds have you been trying? I would keep trying to find a good ground if I were you, maybe sand back some paint if it's stopping a good connection.
What about your battery? What's that like?