I am retied ocean engineer, owned a company that designed deepwater floating platforms for the oil and gas industry. I recently purchased a Beneteau 40 "Hoapili" which in Hawaiian means a close friend. I began sailing/cruising in Hawaii in 1973, owned a 40 ft trimaran, and I am now reliving the dreams of my youth.
I have cruised all around the Hawaiian Islands.
Eventually, I plan to take the boat to the southern Carribbean.
Pieter, there is a 10Amp fuse that also is connected to the battery isolator for the house bank. It's located in the battery compartment under the seat in the aft cabin. Look on the right side near the back and in the vicinity of the rear most isolator. It's possible that there is some corrosion or other issue. Reseating the fuse may help. When you turn on the ignition switch you'll energize both the fan and the isolator.
Good luck and let us know how you make out.