Spedometer & Odometer Don't Work

I am having a strange problem with my spedometer in my 91 s10. Most of the time it works, but if it rains or gets damp outside it will either stay on 0 or go from 0 to 85 nonstop. I went to my local u pull it junkyard and ripped a cluster out of another 91 s10 and put that in my truck, and that one does the same thing. Anyone ran into this before, I can't keep buying parts I don't need. Both clusters have roughly the same mileage on them, and I know they can't both be broken.

Gauge or electronic cluster?
I would have a look at the speed sensor and wiring.
They are what gets wet when it rains.
MT