I like to run the furnace fan to circulate stale air, or even out the temperature in the trailer independently of the furnace heat operation, so I ran two wires to the furnace control board, tapped into the hot wire (Red on my board) and the fan wire, typically Green.
I installed a Red rocker switch on my lighting control board for it. We use it all the time. I also added a Red indicator light to show when the 'Scare' light it on, (which is almost never on) I don't want to shine it and bother my neighbors. Hate it when my neighbor leaves theirs on all night.
I also added a Green indicator light to show when the outside door, or porch light is on. There is a switch for a small light for the bottom step also.
Furnace fan switch install