There have been threads about this in the past. I've asked the same question. It seems all we can do is clean all electrical contacts, clean grounds, and replace bulbs since light output does vary with age.
I wonder if the light switch dimmer rheostat may need to be cleaned in some way, but I hate to crack open the switch body and have to reassemble it with tie-wraps. Would spray contact cleaner help in any way if it could be sprayed into the body of the switch? Or would it just gum up the works by dissolving dried up 30 year old lubricant?

Do older 911's have this same dim light problem?