Just to clarify if theres any confusion.....
The transducer typically has three wires, Brown,Blue and Black.
The display generally 5 terminals.....
1= 12v positive +
2= 12v negative -
5 = signal in from transducer
So connect your 12v dc to 1&2, observing polarity, connect the Brown and Blue from the transducer to 1&2 respectively, then the Black to terminal 5..
Some sellers provide a tacho that has three wires, Red, Black and Yellow....
Red/Black is the usual 12v supply, Yellow is the signal connection.....