Fu3 protects the key switch and everything past it. It is possible that the K1 or K2 relay has a short in it causing it to blow right away. The next part in the circuit is the brake switch and accessory switches. Make sure to turn them off and release the brake to eliminate those as causes. Otherwise TP8, 9, 3, and 4 are good point to test if there is a short on the distribution board.
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