Welcome to the forum Durkin!
The Yashica Minister III has a selenium cell based meter.
As with most of them, the selenium cells do die after some (say 20+) years and the meter doesn´t react to any light. This happens because the combined action of humidity, lighting conditions during storage, temperature, etc.
So, if the meter itself (the moving coil instrument) is OK, probably what isn´t working anymore is the cell. Due to it´s particular circular shape, unless you get another good one from a donor camera, the repair would be very difficult or just impossible.
Here is a guy who knows a lot about Yashicas, Greyhoundman. Perhaps he, or other RFF members may direct you to a repair shop able to carry the job.
Another solution is to get a handheld meter or use the sunny f16 rule which if used correctly, works almost allways.
Cheers.
Ernesto