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hello Dan, Pulse has pretty much hit it on the head, check if your wired positive or negative earth & what alternator you have - take a pic & post on here if your not sure, rest of wiring is fairly straightforward, but depends on the above, regards Geoff
If your alternator doesn't have a W terminal you can usually delve a bit deeper and solder a phase tap to one of the poles before the diode pack, I've modded alternators for tacho pickup on my last boat.
If you don't like what I say don't hurl abuse at the boat, I've sold it...