On my 2012 R27, it was corrosion in the open end of a heat shrink butt splice. During manufacture of the boat, the heat shrink was was not activated on the pump side of a butt splice that was carrying 12V to the washdown pump. That allowed the wiring to corrode and fail. I found it by tracing the 12V wire from the switch back towards its source. I didn’t have to go far. The failed butt splice was in wiring enclosed in some split flex tubing about 18” from the switch. The same failure could have happened in the ground side of the wiring because the ground wire also had an open ended butt splice. I replaced both butt splices.
And as a reminder, 12V appliances like pumps and such require a lot of current. It’s not always going to be a broken wire. It can be corroded connections like ring terminals or switch terminals or poorly crimped butt splices in either the 12V or ground wiring not allowing sufficient current to flow. One may be able to measure 12V to an appliance, but resistance from bad connections will not allow sufficient current to reach the appliance and make it operate. Example: One appliance on our boats frequently cited as inoperative are the dual trumpet horns. They require 8 amps of 12V. My boat was manufactured with wiring to the horns one gauge undersize than called for by the horn manufacturer. That combined with another poorly crimped, but intact, butt splice in the ground wiring did not allow sufficient current to flow to the horns. I replaced butt splices in both the 12V and ground wires to the horns and then they were loud and clear. FYI: Those wires are under the stack tunnel and not easy to get to! The mast has to be removed.
