If you decide to use some kind of photoetched detail set, then I suppose that's best. But, sometimes they're just not what you need. I always paint the back of the panel whatever color I'm gonna use, then use a artist's pastel pencil to
gently scrub over the raised detail. I think I may have picked that up from some magazine or something...and I think it works great. It gives it just enough detail that it seems like you spent an hour painting it. It also helps to paint the needles on afterwards. Thats actually pretty easy in comparison.
Another way, is to "rub away" the paint on the raised parts. Most models are made in a light plastic color. Sometimes (if the gauges are accessible) I'll paint the whole panel, then, before the paint has set up, rub my finger over the panels. That eleminates the paint from the numbers and such. Kinda just based on what you have in front of you.
Yet another way, is to load a brush up, then turn it horizontal to the panels (again, if this is practical) if you lightly, and I mean lightly, drag the brush across the raised numbers, they should pick up just enough paint to highlight them.
Good luck...a great dash REALLY adds to how good a model looks, even though most of them are so hard to see inside a car.
Personal pride though!
-jonathan