Jay,
Two other thoughts:
Folks who own later cars with rear-window defrosters in the hardtop have a control for the defroster in their center tray, aft of the ashtray. You could adapt one of these mountings for the fader, and it would look very original (if you have a 280, anyway). (Now that I re-read your post, your fader might be too long for this idea to work.)
My 230SL has a switch under the dash on the far left (above the hood release) just attached by an "L" bracket under the dash. (No one seems to know what mine was for; supposition is that since the car was originally French, it was a horn selector.) Anyway, this location is very unobtrusive, yet handy.
-David Pease
'66 French 230SL