Ok, I'm reading all that I can on these and I must be totally missing something.

3-prong deal off engine into amp, 3 wires out of amp, green/yellow black/red and some brownish color. Brown is ground (I know that much.) The best I can tell the green/yellow goes straight through to the back plug for the tach. Not sure about the black/red, it goes into the fuse box and I can't really see if it goes somewhere else after that. The plug I got off the tach has 4 wires on it but 2 prongs, wires are green and green/yellow one side and black/red and blue/black (blue/black goes to a light)

Ok so wtf? I've tried just about every combo I can think of. I just had wires run from the battery for ground and 12v to try to get the tach to work then I was going to wire it up. Can someone tell me how to do this? I've searched and found a few threads on this but different color wires were described, so sorry.
-Nate

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'82 240D, 4-speed, 617 turbo engine