I think so. I pulled the cable out of that connection to put into J16.  
So your telling me there should be one more of those connections?

I really appreciate the audio cable. Let me know what it will cost. :)
Once we sort this out the car is the only thing left i believe

Pretty sure both banks of drop targets have an opto board attached and both need the 12 volts to work.
This cable maybe connects between the two.   Should have a perfectly mating connector some where, which is possibly attached to the other opto board.

No worries to do the cable.
Probably looking at about $10-$15 posted.

Now that you can get into diagnostics, should make troubleshooting the policecar a little easier.
Also I'm gonna take a guess that your "belt" is not original.  I thinks lots of different substitute belts have been used by PF owners over the years.  
But as long as it's not slipping or overnight, I guess it's probably OK.