From memory and I am sure someone will correct me the tacho will plug straight into those three wires coming out of the control box.
Green- ac tacho signal
Yellow- switched +
Red- I guess non switched +
I think I also have a black coming out of my box that is -
I have a really vintage tacho that only plugs into the signal and ground wires but all of my previous ones take three, +,- and signal and some take the pink too.
And if a pink is coming out the box it is for the warning lamp or horn (mine is in the box)
Are the connectors in the engine end not connected to anything?