factory tach question

[love it loud]

I pulled the instrument panel out of the cab along with all the wiring and I want to use just the tach in it for engine break-in. (have separate mechanical oil press. and water temp. gauges to use, but no tach..) Question is, there's a three-wire plug on the back of the tach, and the distributor only has a one-wire plug for the tach.. The only thing I can think of is that maybe the other two wires are a power and ground? Not sure exactly how the factory tachs work.. if they're just energized from the distributor or if they need a separate power and ground. If so, could i just run a power and ground wire directly to the battery for now? Just need it hooked up to watch the rpm's during break-in. Thank you for any knowledge bestowed upon me regarding this subject!