How to fix 66 Cuda Tach?

I’m working on a 66 Cuda, where the tach sending unit is remote, behind the drivers kick panel.
I started my debug adventure with a new RTE sending unit. The sending unit works great, it’s output voltage increases with engine RPM, but the tach is still dead.
So now I removed the tach from the dash, and tried applying a 1.5V battery across the two tach terminals, but still no needle movement.
Just wondering if anyone has experience with opening the guts of the tach? I see that Classic Industries has new tachs for $250, but it would be nice to fix the old one