It's weak (and dirty) bulbs but more accurately it is the silver paint they used at the factory on the "light tubes" tends to get old and flake off. It is designed to reflect the light into a clear plastic tube and up onto the instrument cluster. No silver paint = no light reflection. Once you can search, there are a couple of good threads on the topic and I think I posted a picture of the repair process. Some folks have used aluminum foil, others have upgraded/modded to and LED version, but then the dimmer doesn't work with the LEDs unless you add some additional electronics. Pulling the cluster isn't difficult, but a little tedious your fist time.
By the way, welcome to the best place for 968 info!
- Darryl