I have to say, this is a new one on me. I have heard of people having issues with dirt/dust buildup in behind the gauges, but I thought that just caused them to be not as bright. I would be pulling the cluster and checking things out.
I will say that I recently found my issue with dim lights (some not working at all). The little twist-lock bulb holders lost spring tension and weren't making good contact with the board. You can buy new ones really cheap.... around $12 for 10, iirc. I'm presuming the 87-93 uses the same bulb holders as the 79-86, so you might double check that. My point here is that since you'll have it apart, you might consider replacement. The led bulb upgrade is cheap too, and makes them look much better.
If the problem is with the gauge itself, you might consider some new overlays.
Give your Mustang a sleek, white-face gauge look with these 5.0 Resto white face 85 MPH gauge overlays!
lmr.com