There was a Adam Harper converted c5 for sale not long ago. Speaking to the guy that sold it on ebay but registered a thread here on the forum.
He advised that the 12v worked as normal but relays kicked in and sent 24v to the motor on the push of a second switch (instead of the horn switch). It also turned off the POD so no display or status.
It acted as a turbo boost than a controlled / ramped speed.
Lance