Try installing another fuse. Get to the light harness to the rear light(s) and hold the wire. Have someone mash the brakes, It will get warm - hot before it blows the fuse.
I once traced the same trouble out on a 41 that would blow a fuse every time I turned the lights on. It was a frayed wire going to the left tail light in the trunk. This was back in the seventies when I was driving the project 41Hudson, IL to NC . Had to search it out at a gas station in TN. I blew several fuses before I discovered the short.
I only found it owing to superior mechanical and electronic skills. Yeah, right!
Good luck with finding the trouble,
Best,
Charlie