Originally Posted by EarmarkMarine
You can also leave the RCA cable plugged into the existing amplifier, unplug the RCA from the HU, and short the left to right RCA at the HU end with a female to female barrel.
Do you ever use shorting plugs at the amp(s)? I have some from when I sold home audio- Sony supplied them for the unused phono jacks on integrated amps/preamps that had two phon inputs, to decrease noise pickup. I used them as an additional test if I found noise on shorted cables, just to verify/eliminate the cables as the source.