I've two Sirius satellite radio units. One is dash mounted with the
siruis box a hide away unit. That's in my Westfalia which has many
places to accommodate hideaway units.

My other sirius unit is one of those FM transmitter types. I mount
this to my (loose) center console that holds miscellaneous things and
also is a drink cup holder.

In the Westfalia I spent time (as the unit is dedicated) and located
the antennae under the front luggage carrier as the fiberglass is
invisible to the frequencies used.

On the other one, I have the the antenna sitting just forward of the
rear hatch, routing the cable in through the rear hatch behind the
panels on the left side of the vehicle, emerging behind the driver
seat, past the parking brake lever to the center console.

As to purchase places, mostly I've gotten my stuff at Crutchfield's
discount store (physical store...the one I visit is in Harrisonburg,
VA.)

The questions in your 2nd paragraph. No (the vehicle previous tried
was larger and still wasn't able to.) Unlikely...and to the last
question, probably, as Jimmy Buffet didn't change any from owning
(and then not) a Syncro adventurewagon.....