In emacs24 -q,

    C-h f timidity

doesn't have a link button to the timidity.el source code, where I hoped
it would.

/usr/share/doc/emacsen-common/debian-emacs-policy.gz section 5 part E
asks lisp add-ons to symlink the .el file to the same directory as the
compiled .elc.  Recent versions of emacs expect the .el to be together
with the .elc to be sure it's the right source.

So in /usr/lib/emacsen-common/packages/install/timidity do an "ln -s"
instead of "cp", then leave the .el link there (to be removed by the
remove script).