To answer my own post, I did find a perfect spot for the box. It was, as the others said, under the passenger carpet. On my car, there is a sheet metal shield panel which is located under the carpet, not on the floor but on the angled-up portion of the footwell. This panel has a warning sticker about disconnecting a red connector related to the airbag. One screw removes that panel. Undo the screw and lift the top part of the panel and a clip slides out. Easy.

Under the panel is some sort of electronic box with Becker on it - presumably something for the radio (?) mounted on a sheet metal base and a red connector which is presumably the one referred to on the warning sticker. Anyway, the sheet metal panel there has plenty of room for the Sirius box and several holes already drilled. I used one of the holes in it by opening it up slightly, drilled one more hole, and it's a perfect mount for the box. Now the challenge of running the antenna cable!

Rgds,
Chris W.
'95 E300D