my harness is one for factory gauges.i had a brown wire that went to the coil for the tach.i put a female spade end on it & ran it up to my hei cap & hooked it to the tach terminal on cap.i have a factory aftermarket oer tach & its right with my timming light tach readings.
If your car came w/ the factory gauges and tach, you won't find any "extra" wires. The brown wire is in a harness. It runs from the multi wire plug in the back of the instrument cluster to the fuse block. In the engine bay, the brown wire runs in a harness from the fuse block over to the coil.