You could try my favorite, the PIE MPSS-4 source switching relay. It switches not with a button, but based on which device is sending audio.

Two of my installs use it this way:
- The empeg is powered on all the time.
- When the empeg plays, its audio goes to the amp.
- When the empeg is in standby, the radio/CD player turns on and sends audio to the amp.
- Whenever a button is pressed on the empeg to wake it from standby, the radio/CD is turned off instantly and the empeg has control of the amp again.

If you can find an amp turn on lead on the stock radio, than this will work even better. If not, the amp will be on all the time, which may cause pops.

I'm having a bit of trouble with two MPSS-4's I bought recently. I'm working with tech support to resolve it. If you'd like to try one, I've got an extra and could sell it to you.