Coin Door Wiring Question

[Coin Door Wiring Question]

Hello,

First let me say that yes I did do a search and have read the Real Bob Roberts wiring page. I'm hoping that this will be an easy one for those who have done this before...

I have a standard arcade coin door and I want to wire a JAMMA wiring harness so the lights, coins, and meter functions. I understand the 12v and ground, but have a couple of other questions

Here is a picture of the back of the coin door:

 

 

Look at the 5-wire plug in the top right of picture. The top 2 wires are run to the meter plug below. The black is the ground. The bottom wire is power. What is that gray wire for?

Here is a closeup of that top plug:

 

 

Also, volume, test, and service controls are mounted just inside the coin door:

 

 

 

Do I need to attach any wires from the JAMMA harness here and if so, which?

Thanks,
Rob