Lots of possibilities: lamps that are dimmer, caps as above, tape in the lens, variable resistor in the power feed. But I don't know what that dash is like: Are they LEDs or incandescant lamps? Can you add a photo of a lamp and a lampholder. Is there a single power feed at some point? Do you have or would you like a wiring diagram?Jonathan PS: Lots of cars had variable intensity illumination, usually with resistors in the power feed. But the Sunbeam Stiletto had eyelids that you could flick to dim.