After market stereo - Infinity Gold

I recently installed an aftermarket stereo in my JGC and the amp works fine for the CD changer and radio.

However, the head unit also has an AUX input (RCA jacks) that I was hoping to use for my iPod. When hooked up, I get sound only through the speakers in the back seat, and no volume control. I can adjust the volume with the iPod, but something is clearly wrong.

I figured the amp was not powering in AUX mode, but the antennae stays up, and it draws power from the same lead as the amp (I believe).

Anyone have a similar problem?

At this point, I've decided to go with no AUX input, but it bothers me.