I replaced the original tacho in my Plus 2 with a modern 'electronic' type.
The earlier type of tachometer shown in the workshop manual had the ignition supply to the coil looped through the tacho, which was driven by the current pulse.
The later type 'electronic' tacho is driven by a voltage pulse from negative side the coil, and requires a return wire to be fitted for this purpose.
You need to establish which type of tacho you have.
If it is the old type, there will be a continuous input and output. Check this with a test lamp
If it is the new type, the red wire would be connected to the positive supply from the ignition/ fuse 3 towards the rear of the centre fuse box, most probably the white cable, check this out. The existing white ( + supply) would still be connected to the coil, but also connected to the black and white wire to take the supply on to the coil.
The yellow would be connected to the signal from the coil.
The tacho will need an earth connection, either a terminal or an earth tag on the casing or clamping bracket.
Check everything !
Hope this helps