You could use the cord that went to the furnace
but I installed a new power converter so I ran all new wire from that. I put a toggle switch up high on the wall on the left as you come in the door. You'll have to run a ground wire to the light
too. I used a Grote #60311 utility light that is commonly found on the back of the cab of a truck tractor for the driver to use at night when hooking up to a trailer. Nice chrome light but expensive, and the flange is not quite big enough to cover the screw holes from the furnace vent so they will have to be filled, or like I did made an adapter ring of flat aluminum. I think Vintage Trailer Supply sells this same light as a porch light
for Airstreams. It is overly bright so I put a smaller bulb in, 1141 as I recall.