Maybe you should try swapping left-right or front-rear channels and see if the dead speaker changes too?
(though, I don't know how you wired it)
If the output for right front has broken loose internal to the head unit you have to send it for repair or open it up and do it yourself.
Jim,
Lil'Red is a '87 F250 HD, 4.10's, 1356 4x4, Zf-5, 3G, PMGR, Saginaw PS, desmogged with a Holley 80508 and Performer intake.
Too much other stuff to mention.