I hock up loogies, I hook up electronics.
The aftermarket tach requires a specific kind of input particular to that manufacturer's design. Check your tach's documentation. It may require a wire clamped to a spark plug wire, for instance. Depending on how it's hooked up it may need to know you have a 3 cylinder engine.
Read the instructions - it should get you further than us trying to hock up ideas.
If you haven't chosen a tach yet, let us know that too and what it is that's important to you. Price? Looks?