High Level Speaker Inputs to AMP

are high level speaker inputs from reciever to amp a breading ground for alternator whine? i'm dealing with an OEM factory radio with only 2 amplified channels of power, right and left, feeding the right and left front, and i've tapped extensions from these speaker terminals to feed the high level speaker inputs of a cheap 2 channel amp to drive another set of 6x9's in the rear.

and no, the OEM radio in the Ford is a work truck unit, replacement is not an option here, the amp cost just $15, 25watt/channel and i'm just trying to drive another pair of speakers for the rear. they will drive just tapped into the factory harness, but of course, the signal strength is less than desirable and volume is not loud.

ground termination is good, i have used 8gauge for power and ground, both leading directly back to the battery terminals, at a length of 10 feet or less i'd say, and my volt/ohm meter read .7 ohm resistance upon measurement.

i would guess this whining in the rear speakers only comes from perhaps the remote wire turn-on, as i have used a switched power source from the original factory radio wire harness. i will do more trouble shooting.

thanks for any input, Brian-