If you do it correctly, you should end up with something like this.

Now we get to the Fanbus. The fanbus for this setup will be a little different from the usual one that you've seen. The difference is that instead of having 4 different positive lines, it would have 4 different Negative or ground lines. Reason being that the 12V line is now the "common" line for the fanbus. We are now using the negative line to provide either Ground to the fans for 12V, or 5V line to get 7V.

You will notice in the top pictures that I used a male Molex connector for the Fanbus, instead of the usual crimp wire connectors. The Molex connector provide a much better connection to the Baybus and the terminals are also protected within the Molex. As for the line outs of the Baybus, I'm using a Female Molex connector.

Female & Male Molex connectors
Click to enlarge

Don't forget to mark out the wire numbers on the connectors so that the switches on the Baybus will control the correct fans when the units are attached together.

Crimping of wires for the Molex terminals Click to enlarge

Mark out the connectors! Click to enlarge

The other Male connector is attached onto the line in wires for supply from the PSU. And since the 12V line is the common line, all the resistors for the 12V LED is connected to the "common" line too. (see bottom right pic)

Line in connector to PSU Click to enlarge

Both units attached together. Notice the resistors for the 12V LED (right) Click to enlarge

Connect both units together, taking note of the wire arrangements of the Molex connectors, and you have the completed Baybus Control Unit.