I have the exact same problem. Not sure how mine got that way. Putting the battery charger on the battery reverse polarity maybe?
I sent my panel to Upfix.com as they claim to repair these types of panels.
DO NOT send them anything. They quoted 400 to repair. They don't have any reference material and actually did not want the tractor schematic I offered.
I was able to find a few diodes and a regulator shorted which I'm told were replaced but did not solve the 5 amp fuse blowing soon as the key is turned to on.
Rather than try with info from the tractor schematic they just gave up and returned it.
Is there any way to get a print for the panel from somewhere? Kubota maybe?
Has anyone tried that? I'm an electronics tech from way back and would have no problems finding the issue if I had a print.