The Firestorm was a dealer special for Barrons caravans and should be the same for switch positions as the standard models. Eldiss do a back catalogue of handbooks on their website but it starts at 2000 which is unlucky. I would however look through these and see if you can find the answer as it probably has the same switch although I cannot guarantee this.
If not found can you post a picture to see if anybody recognises it.
I did post this before Damien's post appeared on my screen. Thinking back to our Eldiss I am sure he is correct, but if you still have a problem then check the handbook to see if it helps. It might work too for other problems.