This may be what you are after. It all depends on how Mandriva setup kdm. Some distro modify things and makes it a pain to get it right. I don't use Mandriva but currently Centos 4.4. This is the way it works on my system.
The kdmrc file on mine is in /etc/kde/kdm/kdmrc.
Inside the file it Sessions points to Session=/etc/kde/kdm/Xsession
Now this script redirects itself to /etc/X11/xdm/Xsession
In that script I have a line like this after the gnome and kde ones.
exec -l $SHELL -c "$DBUS_LAUNCH $SWITCHDESKPATH/Xclients.fluxbox"
Now that points to /usr/share/switchdesk/Xclients.fluxbox
That script looks like this.
# (c) 2000-2004 Red Hat, Inc.
WMPATH="/usr/bin /usr/X11R6/bin /usr/local/bin"
for p in $WMPATH ; do
[ -x $p/$WM ] && exec $p/$WM
This is the way I beleive it works on my system reading throught the scripts from the start. Hope this helps.