I wanted to share my new found knowledge concerning Mamiya M645 back and it's dark-slide interlock system. I have a back which this interlock to prevent shutter release when dark-slide is present doesn't always work. Strangely enough, if I push the blue dark-slide handle UP, it works. So I took it apart.
To take this apart, unscrew all black screws. Be aware there is one on left side, right above a pin is different. This one is 3 times longer than all the rest. (which are all the same)
Now, once I opened it, finding the reason for this fault was easy. There is a tiny switch right below where 4 pin gold connectors are. This switch is actuated by a small cam which in turn, is actuated by the dark-slide. The switch is mounted on a small printed circuit board with two very small screws. Unscrew these and the assembly comes out. Normally, when the slide is in, switch makes contact. One of the arm had a solder joint come lose (cold solder), so it didn't always make connections.
The switch contacts are connected to the two left most pins. There are three wires connected to the two right pins and one that is shared with the dark-slide switch. In other words, the second from left pin is shared. These wire look to be connected to the ISO dial.
I hope this information is useful to somebody.