even then, the voltage gauge in the dash is wired to the front battery only. if you ground is bad (like being on top of rhino lining) your rear battery isnt "hooked up" you arent seeing voltage drop because your rear battery isnt in the same wiring line as the front and voltage gauge itself. your rear amp worked for a few minutes using only what the battery had stored power wise. it will go into protect when it reaches below 9.6v. IE your battery in the rear is drained with now charge coming from the alt because no ground.