check fuse(s) on your front to rear power wire run. If it's not that, then it's this- You have what i try and preach to people 10,000x is we, car audio community, cannot just rely on the framework of a vehicle as the "perfect" ground. You are experiencing a full 1 volt drop from front to rear. You want to test this for free without installing anything? Get you a set of jumper cables. You need only use the ground side. Attach the ground clamp to ground post on D3400. Attach other end to ground post on stock battery, alt casingbracket or as close to the battery or alt ground as possible. Your voltage should instantly skyrocket. Ifwhen it does, you need to run a ground cable from front to rear of the stock batt to rear batt ground.