OMC tach connections on older Johnson
 

Can someone point a newbie in the right direction...
I picked up a boat with a 1971 60 horse Johnson outboard.
The tachometer is present but not connected. There are two wires on it, one
is connected to ground, the other is not connected. It's an OMC tach, looks
like original on the boat, which is also a 1971 model.
To what point on the engine or control box can I attach that missing wire to
restore the tach function?
Any advice appreciated

Craig