On my 1987 rx7 that I am doing a s5 turbo II swap in to and I am hooking up the wires and on the coil that is closes to the driver side head light. I hooked up the one plug and the other plug I have no plug on the wiring harness that looks like the picture I have, what is this plug for. The coils are s4 it's the little black plug

 

It provides a tach signal and it's for hooking up a tach for diagnostic purposes when working on the car.