Ugh tach only works when lights are on!
 

Took entire front harness out this winter to repaint boot area. Got all back together and everything seems to work except - tach only shows RPM when lights are on. Haven't started chasing wires yet but thinking....is the light switch closing a ground to turn gauge lights on? Or is it allowing 12v?

Car is originally 74 however I have done too much to it over the years to even remember all myself.

Sounds like you swapped gauge B+ and gauge lighting connections on the tach. Is the tach gauge light on when the ignition is on? Should be a 10 minute fix.

Don't think that's it, plus no, gauge light is not on when ignition on. Gauge light works as it should.

Red/Black and Purple/Black are the only two colors that should go to tachometer input tabs.
You didn't happen to connect a spare Blue/Black instead of a Red/Black on there did you?

Pull it and take a look. (I know, steering wheel probably needs to comes off too...PITA...)

I'm thinking something is wrong with tach itself. Voltage from red/black, signal from blue black are same whether lights are on or not, whether hooked up to tach or not. Tach always shows rpm when lights are on and usually but not always reads only zero when lights are off.