If it's a 3 position switch with left to right positions of ACC - OFF - ON, that might explain why the tach doesn't work if it's hooked to the ignition terminal and everything else is hooked to accessory terminal. Does it run with the switch all the way CCW?

Kevin
1951 Chevy 3100 work truck
Follow this saga in Project Journal
Photos
1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car)
Busting rust since the mid-60's
If you're smart enough to take it apart, you darn well better be smart enough to put it back together.