This is a tough puzzle for me nowadays.
   I'd enumerate the paths of connection and judge if this is reasonable
   to do with 2 relays.
   Motor => Forward wiring
   Motor => Backward wiring
   The two wirings can't share an unswitched connection to the motor, as
   this would cause a short.
   So, to do this, four current paths need to be involved in switching. If
   any current path is unswitched, there is a way for it to short when
   something else is switched. I think!
   Then, if it is two relays that guide it, is there a way to turn it off?
   Couldn't you set them to opposite values? Or not?
   What if you turned the relays around? So the common pin went to the
   motor, and each pin was switched to positive or negative.
   Then you'd switch on the direction you want it to go, and it would go
   that way! When both switches were equal, it would not move.