I am getting ready to install a gooseneck microphone... I'll be able to put it as close as I like to my mouth. This will be used in a convertible and I'm hoping to defeat the ambient noise by turning down the volume on the mic and putting it close to my mouth.

I don't see any way to control the microphone input volume via the AVIC. Am I missing something, or should I figure out a way to install analog volume control in-line between the AVIC and the mic?

You're not missing anything...In disguise of user-friendliness, manufacturers don't bother adding options to their products. Oh well..