The header pins. The way this works is that the digital 12V from the PDB (NOT to be confused with the unregulated 12V on J116-J118) are connected to the CPU and the switches via J205 on the CPU board. The CPU compares the voltage on the header pins against 5V - if it is higher, the switch is deemed open, if it is lower, the switch is deemed closed. This happens, when you close the switch which connects the header to ground (there is a "pull up resistor" in there to keep the fuse from blowing when the switch is closed). Ground is provided on pin 10, 6-9 are the direct switches on the coin door. These are called direct as there is no switch matrix involved, all of them are monitored directly. Most game switches are in a matrix, the corresponding column and row signals are on the connectors to the right of J205. We don't worry about those for the time being.
So remove J205, take a cable, connect one end to pin 10 (or the ground braid IF THAT IS CONNECTED PROPERLY!) and touch the other end of the cable to pin 6-9. This should emulate the switches on the door being closed. Report back
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I assume that connecting J3 to the coin board didn't get rid of the fuse error message? The other end should be on J212 on the CPU board, this is the connector above J205. What measuring equipment do you have?