1985 Jetta with non-working odometer?

I'm new and happy to have jumped into the VW world!

My 1985 Jetta, gas, automatic, has a non-working odometer.  Everything else, including the speedo, work fine.  I swapped the guage cluster with another one with the same situation, everything working but the odometer.

Is it likely that I'm unlucky enough to have two bad clusters?  Or is there something else that I can check?  I have no idea where the odometer gets its signal from.

Any help is greatly appreciated!

I've got an '86 Jetta that stopped around 198,000 miles.  It works very intermittently.  My '89 Audi 100 had the same problem crop up shortly after hitting 220,000 miles.  They simply wear out.  Quite common from what I gather.