Quoted from snyper2099:
Cool, how did you make it?
Very cool indeed!
In fact, I just received a little VU meter kit (http://www.vellemanusa.com/products/view/?country=us&lang=enu&id=350543) of my own just two days ago, with the intent of modding my Met in this same way. I wasn't sure where to mount it until I saw this thread; from the OP's pics it looks like it was factory installed - well done!
As far as how to make it work, simply wire the meter in parallel with the cabinet speaker. The VU kit has a speaker-level input (470k impedance), so it's just a simple connection to the speaker terminals. The meter requires 12VDC, which is readily available in the auxiliary 12V cabinet harness. Given the high input impedance compared with speaker impedance (for parallel resistances, 1/R1+1/R2=1/Rtotal), the additional load presented to the game's audio amp is miniscule and won't cause any stress, damage, or audible distortion.
Some meter kits might provide only a high-sensitivity line-level connection. In that case, a resistive voltage divider could be used to pad down the speaker-level voltage to line-level, with the individual resistances being determined by a couple of Ohm's Law calculations (the same circuit some audio equipment manufacturers use to add headphone jacks to power amps/receivers).