Basically, pretty straightforward. Improve the amount of reflection of light down onto the panel by cleaning up the dirty reflector(s).

Remove cluster and take off the back.
I've only worked on '86 and later, has the electronic speedometer.
If your speedo works via mechanical cable things might be different.

Anyway, the bulbs live within a white plastic chamber which reflects the light down towards the gauges etc. Except the plastic gets all yellowed.

I glued a few bits of aluminum foil onto the white plastic part. Just keep the foil away from the bulb socket, you don't want it to get in there and short out.
Illumination is much better with this minor mod. I used Pliobond, one of my favorite glues. It's a removable-bond contact cement, very long lasting. You could use just about anything that will bond aluminum to plastic, I guess. Weldwood comes to mind.

Others have gone whole hog and installed LED illumination. There was tons of detail on how to make it work. Costs some money, too. I got enough improvement by the method above that I'm not chasing after the LED system.
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Sven: '89 245, IPD sways, electric rad. fan conversion, e-codes, 28+ mpg - auto tranny. 850 mi/week commute. '89 245 #2 (wifemobile). '90 244 (spare, runs).