Most Barths have at least one. Mine was disclosed to me on purchase. Its history went way back through several owners, rather like an Apostolic Succession. Perhaps the original owner had secret knowledge imparted from the factory on delivery.

Anyway, one way to track it down is to put a hot light on the back until it lights up. Then start pulling fuses until the light goes out. That will not provide the final answer, but will help narrow it down.

Another way is to put an ammeter in series with it and see if the circuit has a load when the switch is activated. That will help narrow it down. If it shows no load, check lights that do not light for burned out bulbs, for example.

And there is always visual tracking of the wire. Color helps. One person can wiggle and jerk the wire while another watches farther along the run.

If there is power to the switch, you can stick a T pin in a likely wire and have someone operate the switch while you observe a meter or hot light. Close the pin prick with tape, liquid elect tape or shoe goo if it is exposed to road spray.

I might think of other ways, and so will other members, I'm sure.

All of my switches were mystery switches. I had to track them down the same way that Bill H. mentioned.

Today I confirmed by way of the Barth erection diagrams and checking the wires, that the lighted switch above the light switches on the wall by the bathroom (labelled in black-on-black - did Barth outsource some things?) is for the 12 VDC heating pad for the FW tank. Resistance was about 18.4 ohms, yielding about 0.7 A, and about 10W. Top wire (green) is ground for the switch light, center is 12 V hot, and the bottom is supply to the heating pad.