If it is a rocker switch with centre off at rest which I am nearly sure it is there will be power on the centre contact(s) (spade connectors)
If it has 3 connections then the centre contact will be +ve
If it has 6 connections then the centre pair of contacts will be +ve and -ve.
Negative -ve is normally identified on a European vehicle by a white cable. The positive could be any colour, even black, which can be confusing.
I am confused how you can identify power on the upper switches but not this one. It is either there or not. Was it the number of cables that put you off ? Be assured, the incoming power would be on the centre contact(s) of such a switch.
The power to the switch is permanent and is from the habitation battery, regardless of whether the control panel is on or off.
Yes there is a relay which is inside the bottom fridge vent, the power to that relay is applied to both the relay and the fridge when the engine starts, which has the effect of retracting the step automatically and also powers the fridge for 12 volt operation if and when required.
If you can identify positive +ve at the centre of the switch, it would be OK to apply a temporary +ve to check the operation of the step. If that works, you have lost the positive feed, either by fuse or loose / broken connection.
I very much doubt if handbrake adjustment could interfere with the wiring as it will almost definitely be between the two floors, although the output from the rocker switch to the step does go through the lower floor but nowhere near the handbrake adjustment.
I may be able to dig out the wiring diagram which shows the cable colours and where the +ve feed comes from if necessary.
If all else fails, use an upturned milk crate till you get to Prestatyn.