1986 vanagon westfalia rear speakers

I installed a new deck over the weekend and have the static issue when I have any fade to the rear. There were 4 wires at the deck. I hooked them up 'one to one' with the deck. Pos and Neg. In the Rear there were three wires on one speaker and two for the other. The right speaker with three wires had two of them grouped together as Neg. I hooked all those up one to one in 'plug and play' fashion using the existing plugs from the old speakers. It seemed very straightforward, small to small and large to large. Can I undo the popping noise without running new wires from front to back? Where in the line does that third wire jump into the mix? I don't see anthing about this mentioned in the vehicle info database. Any help would be much appreciated. It is not very apparent to the eye how the wires get to the back through the cab.

Let me also add that I installed new rear speakers as well. Somehow I forgot to type that.