<<I have the Garmin too and was told it had to be externally mounted.>>

On my Cardinal I have a portable Garmin for which I have "permanently"
mounted an antenna on the glare shield at the very front and center.  It
gets excellent reception as I routinely get strong signals from 12
satellites - I can't imagine the value of a better location than that.  The
Cardinal does have a very steeply sloped windshield - just as does the ES.
That's where I plan to mount the antenna on my ES.  Any reason not to?

Gary Casey
ES project