I dont have a diagram but installed many brush blocks in 120 amp Betas.
One of the brush block wires (either one) goes to the Field wire (usually blue wire in the regulator harness) and the other to a ground after the brush block is installed.
Rex, I did this on my Beta 38 with Iskra 120A alternator, but with a Balmar MC-614. I understand that the MC-618 is identical to the MC-614 insofar as the electrical connections are concerned; the only difference between the two being the communications link from the MC-618 that doesn't exist on the MC-614. The wiring diagram that Beta gave me is attached.
But I want to highlight something that gave me lots of trouble. The brush box in your picture (which looks like my posted picture!) looks identical to the part I started with, and I later learned that was not the latest-and-greatest version. The problem was that, even with the internal connection supposedly clipped-off, (highlighted in red), that internal connection was still be made internally and causing alternator "runaway."
After way too much time troubleshooting (with the help of Beta and the wizard Dale at Balmar), I finally figured out the problem. Beta then sent me the new-version brush box, with that connection run to a separate (and unused) terminal on the outside. To belt-and-suspender it after so much pain, I added shrink tube to that internal connection (that shouldn't be connected in the Balmar application) and put an unattached (and crimped closed to stay that way!) connector on the outside terminal.
