Two things that are hard to see that will cause fuses to blow.
One is chaffed wires where the insulation is rubbed away exposing the conductor that is grounded against the metal. Look for that GI string of wires where is may be touching a bracket or other metal part under the playfield.
Another thing I came across a few times was a bad bulb. Typically the whole GI string would be very, very dim for a while and then the fuse would blow. I found that by doing the 'remove all the bulbs and add them back in one by one' method.
