Howdy EC.
I am no expert, but I don't think there is a separate AM "amp" or whatever. My AM works fine without any input other than the antenna.
I have had 2 similar HU's from Sony (actually still have, one is not installed in a car right now, the other I put in the wife's van) and the one downer was that it made these loud beeps after the power was turned off (key turned off).
It goes: "Beedoobeedoobeep....beedoobeedoobeep" and repeats. It is actually pretty loud.
They state in the manual that this is to remind you to remove the faceplate for security purposes :roll: . But there is no way to deactivate it.
It does not, however, send that signal through the amp outputs, so that would be a last resort.
Perhaps they removed that "feature". I hope so.