Here is a link to a restored 53/54 Heater, You can see the placement on the decals.
One of the photos shows a decal on the bracket that mounts to the dash,this is for Fresh Air vs. Recirculating Air. It does a good job of showing how it is located on that bracket.
Looking at the decal sheet in Tim's link the decal that says "DEFROSTER" may in fact go on the bracket that fastens to the dash in the same manner the same way.
I am sure Tim is correct in stating that no decal would be placed on the heater itself for the defroster.
The pictures are screen shots from a PDF file I have that was sent to me by a guy who restores the heaters. I can send anyone who wants it the PDF file, send me a message.
Link to Pictures of heater. Regards,
Mike