The "remote" wire that triggers the amp IS 12V, but the current draw required to POWER a cd changer will be far too much for what you will get from a HU "remote" wire. The only safe way to do THAT would be to have the remote wire turn on the amp AND trigger a relay, the relay passing a steady 12V from the battery to the cd changer. That would involve either running a 12V lead from the battery, or tapping in to something that can support higher current draw than a HU "remote" output.
