There should be an amp remote wire on the head unit also. Just connect it to that.

I know this sounds stupid, but it's gotten me a couple times. Remember that the AC and DC volume settings are separate, so you shouldn't assume that just because you can hear music in the house, that the volume is set properly in the vehicle. You'll want to move it from -infinity to 0db when you have it running into the aux input of another head unit.