tach help

I need someone to remind me what needs to be done when using a 
positive ground tach in a negative ground car. I'm referring to the 
early large diameter style used in MKI's and II's. I can't remember 
if I can use it as is, or if something has to be switched inside.

Thanks

Gerard
-- 
One meets his destiny often in the road he takes to avoid it.
~French Proverb

http://www.gerardsgarage.com/FS_Garage.htm