I've installed a Sirius antenna in my Westy and my passenger Syncro.
First the Westy.

Ran the antenna under the center carpet to the area under the driver
seat, then back behind the stove cabinet, up behind the curtains.
Behind the shelf, up over the lip at the hinge mount, then under the
canvas where the crease in the roof metal is, around the edge
(outside) of the canvas, under the front luggage carrier. I remove
the luggage carrier and placed the magnetic mount at the leading edge
(centered) of the luggage carrier, put the luggage carrier back in
place. The fiberglass is transparent at the frequencies used.

Others have mounted right at the front of the dash.

On the Passenger Vanagon, I mounted it at the rear, but didn't make a
real effort to route it under the rubber seal. Don't make that
mistake. You'll allow water in if you do. After getting it inside, it
is easy to route it up the driver side of the vehicle behind the
panels, then under the driver seat and up the center carpet to the
console.