Tachometer failed, but MPG and shift indicator work
 

I have an 1985.1 that I've been slowly restoring. A couple of days ago, I backed it out of my garage to give it a long overdue washing. Every gauge worked. When I started it up to take it for a weekend spin, the tachometer no longer worked. The shift indicator and MPG gauge are still working, and all the other gauges appear to be working as well.

I've checked all the fuses, removed each gauge and reseated the wire connections.

I'm getting voltage on 3 of the 6 wires on the main connector (the green and black when the key is turned to on, the blue/green when I turn on the lights).

I'm stumped on what could be the problem. The car is running well, and all the other electronics and gauges are working (had to fix the odometer last month so I've been poking around behind the dash). Any ideas on what would cause just the tachometer to stop working?