Im not familiar with the game but im guessing that the neon is only supposed to come on at certain times?
The -15V you get on a DC test should be fine id say, but you have your probes round the wrong way as it should be +15V, not negative.
is it possible that you have the connector round the wrong way... its important that you get the polarity for the 12VDC correct.
Im also assuming that the driver chip ( transistor array) sits high and gets sunk to ground when activated like all other transistors in a game... so, if you are getting 15V ( which is a little high) then you should be able to light up the neon with this
