IIRC, you said your smartcraft loom is the same as mine.
There's a white wire and a blue wire that are nmea0183 into the gauges.They should have a rubber blanking plug on the end of them, holding them together.
White should be +ve, blue should be -ve, but apparently the merc manual says if it doesn't work, try it with the wires the other way round and you wont damage the gauges.
<edit>(double checked with the PDF-white is +, blue is -.)
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