HSTuners - Cluster install

I have a 92 civic DX and I'm installing a cluster from a 92 si ( my stock cluster does'nt have the RPM Tac) . The problem I'm having is that on the back of my cluster I have 3 plugs and on the back of the Si cluster there are 4 plugs. All 3 of my connections fit the new cluster except there is 1 plug on the Si cluster left open , it's in the bottom center of the cluster and it's green in color. I'm wondering if this is for the RPM gauge. If it is , that means I'll have to wire it up my self, if so, does anyone have access or know where I can ge the wireing diagram. By the way the motor is out of the car so I can't check it that way.