2004 V70R - lost all sound in the back speakers

[Driver5]

The rear audio on my v70r is just barely there, the front left/right sound fine. If the fader is turned to full back there is just a tiny output that is scratchy. I am thinking it is either the amp or the head unit (HU 803) that has gone bad. Anyone else have experience like this? I may try to check output with a multimeter. Thanks for any help.........

Driver5