OK, “backyard” I can almost buy as a noun, since it’s been (fairly infrequently) used that way, and not just as an adjective, since the 17th Century. It’s still infelicitous. (And note the differences in pronunciation between “back yard” [separate adj+noun] and “backyard”[adj]) But using the adjective “backseat” as a noun in place of “back seat” is just laziness, committed by writers whose verbal vocabulary exceeds their reading/writing vocabulary (and who have a tin ear for nuances of pronunciation, as well). How many morons write “frontseat” to be used as a noun? Yeh, maybe a few–though in today’s increasingly illiterate society, increasing in number–morons. Unfortunately, there is a growing number of subliterate, lazy writers misusing adjectives as nouns. . . and editors and “proofreaders” who are just as subliterate and lazy. All should be flogged with dangling participles.
And all of the above who get paid for their abuse of English should be flogged–for real–through the streets before being tarred and feathered.
That is all. For now. . .