tach not working

Just a guess... tach wired wrong. That instruments connectors aren't marked. There is just 2 threaded studs mounted in a little piece of board. These can be poked through the housing in either position during assembly so marking the housing wouldn't be true.
The shorter of these 2 studs is signal. The longer is 12 volt. Although a male spade terminal is added for the signal wire connection, that could be placed on either stud. Stud length difference is the only true key.
If it is hooked up backward and grounding via the 12 volt line, we know a chassis ground is present.
A dedicated chassis ground wire should have been added at lower center mounting screw whether there is a tach in that center pod or not, and especially where there is a tach. All of my rally inst' panel customers knew these things before installing.