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

I have a Garmin GPS 150 and a Garmin GA 56 antenna mounted inside, just
behind the rollover support on my 320.  I have never had a problem acquiring
satellites.  Have been flying with it since 1995.  As long as your airplane
is not painted with metallic paint, you should not have a problem with the
antenna mounted inside a fiberglass fuselage.
Bob Jude