Found this cool, color coded schematic on ipdb.org. A big thanks to whomever put that together.

Wizard credit button (resized).jpg

So does the Credit relay not fire when you push the credit button?
This shows that the Credit button should fire the Credit relay through a chain of five switches starting at the bottom/white wire:
- Score motor switch at position 1F
- Reset relay switch
- Credit button switch
- Credit Unit zero position switch
and one of these four switches:
- Coin Unit limit switch (probably the most likely of the four)
- Player Up unit zero switch
- Ball Count unit zero switch
- Game Over relay switch

You should work through this circuit with an alligator clip test lead or a meter to check that each switch is working as it should.

/Mark