If the light works on shore and battery then the fan will, too.
The polarity of the fan really does not matter. The DC fan will turn in
either direction depending on the how it is hooked up. Customarily the
red lead to the computer fan is positive connection and black negative.
Here are three schematics to consider. Second one ok
but fan and light on together all the time. You could put a second
switch near the fan so you could use the light without it.
The third drawing is a little primitive but it is the Gucci way to
hook up the fan. The fan is independent of the light and can be
used for exhaust or intake. Probably more complicated than you need?
Good luck on your project. Larry H