Have a look at this video here -
It shows the wires going into the step switch (at around the 10 minute mark). The wires going to the step motor are orange and purple in this model (from memory this is the same as my step, rather than red and black as in the Thule manual previously referred to). Pull the purple and orange wires off the switch terminals and apply 12v to them and the step will retract / extend depending on the polarity supplied. There is no need to dismantle the step casing to get to the wires to test it independently. Just apply power to the orange and purple cables behind the switch.
That’s really helpful, thanks. Two of the 4 wires to the step are indeed orange and purple but they go to the relay that I can’t quite get to not direct to the switch and until I manage to remove the black Ducato trim underneath the passenger seat belt I can’t see which wire goes where on the relay or get to the relay to put power to the orange or purple wire.