Follow the wire and check it for a connection at every point/junction/contact it makes. Do you have a multi-tester with a continuity test built in? Most do. Also check for a good ground at the bulb socket.

is it the #2 on the bg. in the 2nd player or the lighting in the 2nd player score box?
-check the 'ball index' relay
try jumping the contacts with the blade of a screw driver (when game is in 2nd ply. pos.)
of the SWs and see if it now lights
-may be hard if the relay has two levels of contacts Hope this helps

"There's a good ground and I can't follow it without cutting alot of wire bundles. If i knew where the coin unit disc was I'd check the voltage there. thanks"

This is a guess, but I think the "coin unit disk" sends a signal to a chip ever time you put a quarter in (in pay mode) or every time you push the credit button (in free play mode). There for being connected to all the player lights, so testing from there wont do any good, it still powers or tells programing to power the other 3 player lights.

From the termolgy being used I'm also guessing the "coin unit disk" is on the coin door. Look and see if any of the wires connected to the coin door match the color of the wire coming from the non-working light. Again, just guessing.

The coin unit will be on the relay board inside the cabinet -a reset stepper motor,
the only thing bigger then it will be the score motor -should be tagged
however if the ID is off you can go by the coil #s on the wiring diagram to confirm.