The harness has 4 RCA connectors - OEM amp only uses the "front" channel since there is no "front-rear" fader on OEM head units. I've got one in my parts stash (adapter) and I'd bet where the other two RCA plugs go into the connector there is no corresponding wiring in the cars harness connector. Seems to me if you had a 4 channel setup.. that DAC unit is only going to play out of the front or rears only.. not both..


OEM amp is 4 channel - but the inputs at the amp end merely split the front channel L/R to feed front and rear speakers.