The blower is wired to the ignition switch and should turn on first before starter and is located on the stern up hi
Having owned power boats with gasoline engines it became habitual to run the blower for awhile before starting the engine, and as part of the refueling ritual. Even though the sailboat has a diesel, I turn the key on and let the blower run before I hit the starter button. The blower is wired to run continuously while the engine is running.
When I first got the sailboat, I wondered why the fan was not wired with its own on/off switch. A 2 cylinder diesel certainly doesn’t inhale and need to exhaust as much air as a 350 Chevy V8 which sits under a smaller enclosure than the engine space in a sailboat...
The blower is off when the ignition is off, and probably should run for a time after the engine shuts down to help cool off the interior of the engine compartment. It does stay fairly warm there