First thing would be to scour the internet to see if you can find a schematic to download. Even if you can find one for another pin in the same family or series, they many times used the same guts as their brethren I typically look at ipdb in their chronological listing to find those built about the same time and deduce which might share guts thus similar/same schematics.

From there its more detective work. If its all the bumpers, most likely the short isn't in one of them, but somewhere further along the circuit after they've been tied together. Then its a matter of collecting data - does behavior change if bumper score value changes, or do pf parts of the same score produce (or not produce) similar behavior, etc - to direct where you might look next. Stock up on fuses. I went thru most of two boxes troubleshooting a flipper coil short in the bad wizard. Once a suspect area has been identified, then I'll generally start looking for any spots that have had prior solder/wiring work as those are prime spots for shorts, bad solders, etc.

There seems to be some good EM folk here, so I'm sure someone can provide insight once you've been able to dig into it some.