Score motor 2C, YEL on one side, BL on the other.

Switch on the Start relay at start; switch on G during game.

The easiest thing: Look for the big blue spark when it should be resetting. Then, you have found your switch!

Other possibilities include making sure the contacts on 2C are cleaned well (they are the large style contacts, and become pitted, so a hard file is sometimes need more than a flexstone), and the position of the coils on the drop target bracket (sometimes the plungers need a bit more plunge travel, if they're hitting the coil stop before lifting the targets all the way).