i've got a HDS-5 combo unit fitted on a ram bracket at the helm of our half cabin whittley, and there's no need for an external GPS antenna in this position.
GPS antenna's (internal) are alot more sensitive and faster than they used to be.
On a cloudy overcast day, i'll still get 7 sat's no problems (and unit does not have clear view to sky).
hope that helps, look forward to your review(s).