Dim instrument lighting seems to be pretty common on 928s. There are three triangular shaped plastic pieces on the bottom of the gauge housing. These have a chrome-like finish on them, and are what transfers the light from the bulb to illuminate the gauges, in a fiber-optic sort of way. One of the reasons that the instrument lighting is dim on many cars is because the chrome-like finish on the plastic pieces has deteriorated over time. Mine were more of a dull primer gray color than chrome. The chromed plastic pieces aren't available separately, so you'd have to buy a whole new gauge housing to really correct the problem. I experimented with a couple of different chrome spray paints, but those didn't look any better than the messed up original coating that I had before I started. The side of the paint surface that actually contacts the clear plastic is what's really the important surface, the same as the reflective coating on the back of a mirror. With the regular chrome spray paints I tried, that surface against the clear plastic was just gray.
Today I found a solution though. At a local hobby shop I came across Alcad2 chrome paint. This is not like any of that other chrome paint you've probably seen. Don't let the way it looks in the bottle fool you. It looks amazingly like chrome plating when sprayed on. The instructions state that you should use a gloss black paint as a sort of primer under it to get the proper chrome finish. Since the reflective surface on the three plastic gauge housing pieces should be facing in towards the center of the clear plastic, I sprayed the Alcad2 on first, and then the black on top of it. The area where the light enters and also where it exits the plastic pieces needs to be masked off before painting. I was really surprised at the final result. It should work at least as well as the original chrome coating, and it cost less than $10 to do.