Trailer brake lights

My brother, with a Ford found this fault, but I thought I'd pass it on.
While towing a trailer, my brother found that his trailer lights blew the vehicle brake light fuse. Then he found he couldn't shift out of "Park".
I know I have to press the brakes to shift out of "park", so I guess you need the fuse too.
I'd hate to find that out some rainy night.