Factory Tach Wiring

The factory tachs do work with Chryslers electronic ignition. They may not work right or for very long with some aftermarket systems like MSD.
The factory tach has a small board inside with both contacts on it. It will pass through the housing in either position so the housing wasn't marked which is which.
My personal method of remembering is short = sender. Both start with S which is the sender marking on most other instruments.
When I add a chassis ground wire to a rally panel it is attached to the lower 1 of the 3 screws at that center pod whether there is a tach, or clock, or just a blank in there.