Only a small amount of the light comes through the gauge faces. If you look at the bases where the bulbs fit you will see most of the light is directed around the sides of the inner body and emerges between the inner, and outer bodies at the top. Effectively illuminating the top of the dial. Some may well come through dial face so you could clean off the old lettering. The fact that you will be reassembling with the glasses clean inside will make a big difference to the illumination.
