2004 porsche carerra 4s radio

I have a customers S4 in the shop. They recently replaced the battery and the radio would not turn on.

I have pulled the unit and replaced the fuse and now have power to the radio, but no audio output.

I have +12v at  the amp, the changer will not eject the cartridge either.

Every fuse in the kick panel has been checked with my meter.

Is the amp blown? Are the Bose speakers 4ohm?

Can I put in a new Head Unit, Changer, Amp combo with a little rewire and be fine?

Is there anything Can-bus related to the car?

There may be a fuse under the wiring harness of the amp.  it sounds like the battery may have been connected backwards, this ususally blows fuses on the amp and the memory circuit for the radio.  The changer not ejecting leads me to believe that there is a blown fuse on the changer too.  Unplug the amp and look for a fuse.  It will be next to the pins of the amp, It may have a black cover over the fuse.  On the other German vehicles, it is not inside the amp.  If you do not see one next to the plug, It may be inside the amp.  There may be more than one fusebox, look in the front cargo area.

i replaced the fuse behind the radio and it turns on and works fine.

Just no audio output and the changer doesn't eject or play.

i have power at the amp so it must be an internal fuse?

i will open it up on monday.

i can't find any other fuses or fuse panels in the front "trunk"

every fuse in the drivers kick panel checks out fine.

Thanks for all your responses.

Cannot find anything inside the factory amp.

I disconnected all the wires to the radio and drove it around for a bit.

Reconnected everything and still nothing.

It has four sets of 2ohm wires and one set of 4ohm wires at the amp.

Is it safe to assume the meter is just reading the combined impedance of the mid and tweet?