reftex-external-file-finders

reftex-external-file-finders is a variable defined in `reftex-vars.el'.
Its value is

(("tex" . "kpsewhich -format=.tex %f")
 ("bib" . "kpsewhich -format=.bib %f"))

Documentation:
Association list with external programs to call for finding files.
Each entry is a cons cell (TYPE . PROGRAM).
TYPE is either "tex" or "bib". PROGRAM is the external program to use with
any arguments. %f will be replaced by the name of the file to be found.
Note that these commands will be executed directly, not via a shell.
Only relevant when `reftex-use-external-file-finders' is non-nil.

You can customize this variable.