One thing worth trying early is running new ground wires to the amp and HU.


/strip away any dirt paint etc before attaching the ring terminal - shiny metal is what you want. afterwards,put on rust protection on top


While you're at it,check the grounding of the battery (negative terminal) and the alternator (commonly grounded to the mount on the engine block, so also check the car body to engine block grounding strap. Beefieer ones doesn't hurt; clean the attachment points)