why are my back speakers whining but not the fronts?

[1meanZ 10+ year member]

Just hooked up a Kicker ZX700.5 and I'm getting alternator whine through the back speakers only. When I disconnect the rear RCA's from the amp the whining goes away but so does sound too of course. If I disconnect the RCA's for the rear at the HU I still get the whining. I've regrounded the HU and nothing. The gain is set at less than 1/2 way. Any thoughts? This only takes place with the car running BTW.

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[Boulderguy 10+ year member]

Sounds like the rear RCA's are picking up noise somewhere. Try a different set or moving the original set around.