One speaker not working with new head unit install

[retroresolute]

Car: Ford Focus 2007 ZX3 SE "Premium Sound Edition" (as far as I can tell, this means that the car came with a six CD changer unit as opposed to a standard, single CD player)

Stereo: JVC KD-R740BT

Wiring Harness: Scosche FD23B

When I first attempted to install the head unit, the display read "Miswiring". I then reconnected the speakers one-by-one until I found which ones were not working correctly. The stereo operated appropriately with all speaker wires connected except for the rear right. I reconnected the original stereo and all speakers played appropriately.

Short of assuming something is wrong with either the wiring harness or the JVC unit, is there a common problem that could explain why the speakers work with the old unit but not the new? If a problem with the JVC unit or wiring harness is possible, is there a good way to test this? I planned on connecting the JVC rear-right to the wiring harness rear-left to see if it recognizes the speaker as valid at that point (at least determining that there is nothing wrong with the connections on the JVC end) but have not had a chance to do so.

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[Jepalan]

You may have a short to ground somewhere in the rear right speaker wiring or terminals - the factory HU may not care, but the JVC does. Check between each speaker lead and vehicle ground with ohm meter. Otherwise it most likely has to be a mis-wired connection between the scosche and new HU (most likely cause). Double check your connections very carefully (purple and purple/black for rear right speaker +/-).

 

[retroresolute]

Thanks for the advice. Turns out the rear speaker was not actually working with the factory unit (I assumed it, too, would have not played or thrown some error if that were the case) but I had not noticed this before now. While this means I have a whole new problem on my hands, at least the JVC unit is working correctly.