Jeff,
I have enclosed a link that has just the info you need.
I think your brake circuit would do fine with 16 gauge wire and a 5 amp fuse.
I don't know what size wire the original light switch has on it. My guess would be 12 gauge wire with a 20 amp fuse.
If you put a inline fuse holder in the light switch circuit (you should), and it blows fuses under normal nighttime driving, you would need to step up to the next size fuse.
Maybe your shop manual would show light switch fuse size.
http://www.crowbarelectricalparts.com/Specifications.htm [edit] The '41 shop manual states your truck has a 30 amp fuse. It appears the only fuse on that rig is on the light switch, So I would say that would answer the size question.
Stuart
