Theory Of Operation
WARNING: This page is about a different car, the 2016 Jeep Wrangler and 2015 Jeep Wrangler. However, it is still accessible from the selected car via links, so may be relevant.
The (X51) Left Rear Audio (+) circuit and the (X57) Left Rear Audio (-) circuit are outputs of the Radio. The Radio controls and monitors these circuits for a concern.