David Letterman interviewed John Keel on July 28, 1980. They discussed forteana, strange rains, creature sightings — and a cast of a Bigfoot footprint. Letterman was more curious than dismissive, and John did a great job. And you can watch it here.
Again, thanks to Anthony Matt and Angela Alverson for technical help.
ADDENDUM: As Loren Coleman has pointed out, the footprint cast in this clip was actually made by Edward Shipton in Nepal. It has a troubled history: see here for photos of it, as well as some of the charges of hoaxing. As was often the case, John made factual errors; the evidence in question is suspect; and he nevertheless made a great case for a Fortean perspective within a mainstream context. The fact that Forteana is riddled with errors and hoaxes just goes with the territory — and, of course, errors and hoaxes are not unique to Forteana!