The last step of the bootup sequence typically results in a switch or switches closing that will connect power to the playfield (PF). Your bootup sequence is not completing, thus no power to the PF.
Yes, you got it; the index cam is one of the end cams on the score motor. Consult the schematic and look at the lower part, and focus a little to the right of center. You should be able to locate the Index cam, and level D is the 4th level up from the follower arm that rides along the edge of the cam. In THIS case, Level D is the 2nd switch DOWN from the topmost switch. If you consult the schematic, you will see that one of the things that the switch at Level D accomplished is to pulse the Outhole relay. The switch itself "may" be a make/break (M/B) switch.
Locate the Index cam. Williams always put the Impulse cam on one end of the score motor and the Index cam on the opposite end. The Impulse cam has 8 or 9 "teeth" to pulse its switchbank, while the INDEX cam will be circular with 2 notches that the follower drops into as the cam rotates. Run a flexfile between the contact faces of the Level D switch to make sure they are clean.
It will be hard to see. It just might help to loosen one end of the score motor assembly and swing it upright on its hinge to try and see it better. Try and inspect to see that the Level D switch is indeed opening and closing the contacts. If you can see them, try to check the tab ends where the wires solder on and make sure a wire is not broken off.
You're getting close. Just stick with it.