I know it's a mildly old post, but since I'm reading all of filmsurgeon's posts to aid in my ragtop restoration...
There's another way to mask & paint the heater controls, using a rubber-based compound to mask the flat surfaces inside the rectangle and around it (and inside the letters). I used to use something called Maskol (made by Humbrol) back when I was painting models as a kid, it's just liquid rubber that flows reall well so you can use it to fill the lows, let it dry, and paint the highs with anything you want (a rattle can, or a brush and paint for example). The layer of Maskol gets painted too, but you just tear it off once the paint is dry.
It might be possible to use liquid rubber products like plasti-dip too (not sure about that, I've only used the aerosol version).

I haven't checked my heater control yet to see if it needs to be redone, so I might try it soon.