You fitted a pioneer unit? Sounds like you may have damaged the ground on the rca outputs, pioneers are prone to dropping rca ground's because of a little fused link on the circuit board, this can usually be blown when a unit is used whilst having a poor ground, the headunit tries to use the rca grounds as an earth, thus leading to problems. To check this you need to disconnect the headunit completely, and with a multimeter check the continuity between the rca shields and the case of the unit.

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