Whining in speakers after installing amplifier

[mercedeslimos]

Hi all,

Got a bit of a problem with my B6.

I installed a 4-channel amp (Legacy 800w) in-line to the stock speakers in my B6.
i.e. cut the wires behind the stereo and ran them to the amp, and back up to the outputs to go to the speakers.

I am getting a high-pitched whining when the ignition is on (and thus the amp - the remote wire to turn on the amp comes from the cigarette lighter),

When driving the whine goes up and down in pitch with the RPM.

The amp has a good solid ground and is located in the factory location under the passenger seat.

Anybody any ideas?

Sam