Unfortunately I don't have any 1950 Mercury parts manuals or reference books, but maybe these illustrations from the 1950 Ford parts catalog and shop manual will give you some guidance. I would assume the basic under-dash assembly process would have been similar for Mercury, but don't know for sure.
The illustration showing the behind the dash view shows where the Ford flasher is mounted and mentions an orange wire with a yellow tracer - that should be the wire that has the fuse holder attached, coming from the ignition switch. It appears that on the Ford, the flasher is mounted up high near the radio speaker. Probably not what you wanted hear, but you might have a little more comfortable access to it by entering from the passenger side. Hope you find this helpful in some way.