I used some white paint-on "wite out" stuff to paint over the top of the bulb. Solved this problem perfectly. I had to do it twice to get the right amount..... took more than I realized to dim the bulb enough.... and if you put on too much, it scrapes right off. I did my '86 about 4 years back, no problems since.

"Wite out" is the stuff used to paint out typed mistrakes on paper. (I tried it with my word processor, but even when I paint out the mistrakes on the screen, they still end up looking stoopid on paper...) In addition to being white, this stuff is opaque so blocks the light.

"...whether they run on 12v DC for example, and whether the brightness could be controlled (resistors?)..."

You'd need a substantial current-limiting resistor. Full 12 volts would pop that sucker in about one millisecond. Yes, you could reduce or adjust brighness using more or less resistance.

"If I remember correctly, diodes have polarity issues, and I'm guessing that if it was installed "backwards" it wouldn't light up. Is this right?"

Right. I wouldn't do a thing.

I think the "wite out" approach is far easier and cheaper than an LED....
--
Don Foster (near Cape Cod, MA)