Hi, the amp may not be wired correctly, you'll probably have to splice the power antenna wire from the radio. One end will come from the radio power antenna out and then be split; one goes to the power antenna and the other to the amp power on, it should be a very small wire.
I did splice a blue wire from the amp into a blue wire for the stereo, which I believe was a remote turn on, so it should power on with the amp. I did get a fairly cheap wiring kit, but it was the one amazon recommended, so I didn't think much about it at the time