The tach adapter is an Auto Meter 9117 (from Summit)
http://www.summitracing.com/parts/ATM-9117/?rtype=10
There are 4 wires on my tach adapter, (1) ground (2 & 3) in and out of adapter from the COP wires, (4) goes into the cabin to the tach for the signal.
The back of the tach has 4 wires...