The standard in the idustry is purple.... I don't see any purple wires anywhere. There should be some on the back of the gauges I would think. I put in a Centroid fuel sending unit and it calls for a ignition hot wire. I have it hooked up right now to blue that is tied to a yellow and it works. But this is the gauge lights I believe and my on the run batt charger is charging back though the gauge lights I think.

Is there a wire at my oil tank that I could tap into?

http://www.centroidproducts.com/data.htm