problem with input method

[giglio robbò d'acciaio]

I've the following problem with the input methods in Emacs 20.7.1:
editing a LaTeX source file it opens it using the "@ -- iso-2022-8bit-ss2-unix"
coding system while I'd need to use "latin-1", but when I tell it to select it
as current, it answers me: 'Can't activate input method `latin-1-prefix''. Why?
How can I solve it?

Thank's in advance
Daniele Giglio