Peter,
You may get away by removing only the radio, unless its the one with the fan switch, in which case the glove box,clock and fan switch have to come off as well. (I think this applies to left hand drive cars as well :oops: )
Loosen center piece: To get access to that nut, move all levers to their outer extremes.Should be able to find the stud/nut by feel thru radio hole.A small mirror can help locate this.Can loosen nut by using 10mm 1/4 drive socket.
Now move all levers to the center (vents closed position)and find the nut for the piece you want to swap, and remove. Lift it off slightly to clear stud and slide away from levers.
As usual, fitting is reverse procedure. Sometimes, its very difficult to thread the nut back on the stud as there is room for only one finger. I used a latex glove and stuck the nut to forefinger.
Once threaded, pull of glove.
Hope this helps.
naj
65 230SL
68 280SL