There is a circuit breaker in the 3 lever light switch...you didnt mention it so I wonder if either:
Someone rewired and got rid of it...
Its there, but malfunctioning.
The power into the whole lighting system is on the wire that runs to the F pin on the cannon plug that connects to the back of the 3 lever light switch. If you take the cannon plug off the back of the 3 lever switch, look real close at the inside of the plug and you will see the letters.
The primary power distribution wiring looks like this:
If you follow the big wire from the alternator toward the foot starter switch and find the block on that wire that is the factory splice...the smallest wire from that splice will go through the firewall and split at another splice. The middle wire on the opposite end of the splice from the wire that you followed to get to the splice is the one you want.
Short....you should already have the proper size circuit breaker in the 3 lever switch....but something is wrong to make the problem you have...I'd bet the 3 lever switch is bad.
Lord send your Holy Ghost into our hearts and make the desire of our hearts Your Will.
Pro-choice, that's a LIE, babies don't choose to die!!