2006 A6 electrical issue
 

I recently purchased a 2006 a6. It ran fine then it started to have a throttle body flutter at idle. I discovered the car was hit in the rear so I started looking in to it. I discovered that when I disconnect the box connected to the ground cable on the rear body panel the idle smooths out. so I thought it was the box. well I replaced it and found out it was not the problem. I also replaced throttle body . not the problem . reset throttle body still runs like crap at idle. checked the rear harness for broken wires found none. looked on all data found a link related to the alternator leaking ac voltge disconnected single wire at alternator car smoothed out with the plug in the rear still plugged in .
I need help please finding a solution thanks .