Remove the brown wire from the 4 terminal side of the switchbox and attach it to 1 of the yellow terminals on the rectifier if your useing a alternater powered tach then just plug in the front of the control box
In 66 a inline 6 with conventional ign. (2 coils) there was no tach wire or extra wire included in the harness from merc as that was considered a price engine at the time. You will have to find a later control with a tach plug included or run a extra wire