The thing about xmodmap is that it only pertains to programs running under X; it doesn't affect e.g. the console or a non-X window manager.
In order to remap the keyboard on an earlier level,
loadkeys is what you want. It uses the keymaps (usually in
/lib/kbd/keymaps/) corresponding to the physical keyboard you're using. Usually you set this up when installing the system so that it uses a map appropriate to your keyboard.
I've not actually done this recently, but here's how it should work if I've got it right:
1 First, dump the current keymap to a file:
dumpkeys > /etc/sysconfig/localkeymap.map
2 Take a backup of that file in case something goes wrong here.
3 Edit that file to change what you want changed, e.g.
keycode 37 =.
4 Verify that it works with
5 If it works as expected, add the
loadkeys command to a suitable start script.
If it doesn't work, use
loadkeys on the backup file you created in step 2.