Study the image closely - it gives wire gauges, NAPA relay part number, and the full "how-to". Note that I put that guide together many years ago, to post on a forum with more posting limits than this one. Consequently it's a little bit difficult to read, and the "purple wire" (from the P.P. unit) referenced  may now be different color. The important thing is that you isolate the P.P. unit such that it's running directly from the battery and is NOT sharing power with anything else. Any power-side connection shared with another component on the "hot" side of the key switch may result in a boat that turns on with the key but can only be turned off by disconnecting the battery - not a good idea. Essentially, the output side of the key switch sends power to the NAPA relay, which closes the direct power lead from the battery to the P.P. unit. When the key is turned off the relay is no longer powered, so the circuit opens and the P.P. turns off. When the relay circuit is closed (P.P. running) power comes directly from the battery and that power is not affected by a drain on the dharging system (from turning on the depth finder, heater, stero, etc...). That relay, BTW, is used in automotive applications to switch electric fuel pumps and other essential components, and has been field-proven to be tough and reliable - which is why we chose it. TW