I am not too sure if it is a question of where the lead wires are connected inside the plug, but I can attest that it rarely works. I have 3 piece of equipment with 12v and they all required for me to find a mono plug. You don't need the whole cable mono (if you already have the stereo in a wall or something), but at least the connector.
In fact, in my setup, only the portion in the receiver is mono (with splitters). 2 out of 3 of the depending devices worked with a stereo ending on their side (but one did not).
