If the previous installer didn't properly install the head unit, then they might have improperly installed the rest of the system. It isn't that expensive to entirely replace the primary 12V+ wire, the negative ground wire, and the turn-on lead. Make sure the ground to the chassis in the trunk, from the alternator, and from the battery are good. You can check them with a digital multimeter. You need a 12V DC relay with fuse installed inline on the head unit's turn-on lead to turn on the amplifiers. That is maybe $10 at an electronics store. Do you know your alternator isn't the cause of the problem?
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