Stick a paper clip or a spare connector into the plug to get the probe on it (obviously with the plug removed) to test the continuity. Yes, you can crimp 2 wires into the same pin if they are smaller gauge or you can solder the 2 together and a short length as a pigtail into the connector instead.

The switch test for system 7 is nice in that it will show all closed switches; do you get all 3 closed or the right most is open? Try taking the 3 balls out and put 2 in the upper lock trough, and the 3rd in the shooter lane - you should be able to start a game from there.

Some of the early williams are funny in that they will not start a game if they see TOO many balls in too many places, not just not enough in the correct places. So if there were 4 switches closed (say the shooter lane, too) it wouldn't start either.

Conveniently any williams system 9 or 11 game that has 3 balls in the trough can be harvested for the needed parts. I'm always torn about doing the conversion because microswitches doesn't mean maintenance free. The real issue with the leaf blades is that they get very mangled and thus hard to adjust. If they aren't mangled you can usually get them working 100%.... but if the contact is reversed that has to be replaced no matter what.