I have a '91 GT convertible. I have noticed that my instrument lights are pretty dim compared to all my other cars. If I turn the dimmer all the way (so that the dome lights come on), then the lighting seems to be just bright enough. I have also noticed that my headlight seem to be rather dim, but I think that is because my lenses are so cloudy.
I tested the voltage output of my alternator, and found it to be about 14.7 volts. That seems to rule out the alternator as a source of the problem.
Any ideas?

 

Your stock alt may be putting out 14.7 volts but the ampers are not there. The stock alt has a horrible amp rating which causes electronics to get their required voltage...but just not power. Try revving the car and see if your lights brighten...if so then it is just your stock alt. If they don't brighten at all....maybe they are dirty.

My instrument lights are dim too....but be happy...yours are even. Half my speedo lights are out....and the others are dim.

 

You have reminded me that I did not test the voltage with the lights on. I retested and found that voltage (at the battery) is about 14.5 volts. This is PLENTY of voltage. As an electrical engineer, I can assure you that the voltage will drop as current through the lights drops.
On top of that, my other cars seem to be in the 13.7 to 14 volt range under similar conditions.
I do know that there is a significant voltage drop across the dimmer switch. I tried replacing that a while ago.
Maybe they are dirty. But looking at my headlights, they are a pale yellow.