So the light comes on without the cables attached as well. There's nothing close to the back of the board at all. And I can't detect any short in the bed wiring either, cables and connections look fine and resistance is 1.8 Ohms, which I guess is normal. Also , I didn't touch anything related to the bed when installing the second fan. Oh, and I run the fans on the internal 5V supply, if that might be a clue?
I have a multimeter but I don't know what to look for if I should check the mosfet and driver chip.