[flyingfrizzle]

Tach diagram:

http://boatinfo.no/lib/mercury/manua...3lit.html#/158

Gauge arrangement:

http://boatinfo.no/lib/mercury/manua...3lit.html#/148

Engine harness:

http://boatinfo.no/lib/mercury/manua...3lit.html#/172
http://boatinfo.no/lib/mercury/manua...-3lit.html#/98

The 225 3.0 is different than the 150-200s of that same year with the 40 amp system dual 20 amp rectifier boxes (looks like what you have pictured in your post) The 3.0 will have an alternator and the grey wire will come off of the box shown as #34&35 in the manual provided in the link above. Sounds like you could have a corroded wire or connector. Check all the connections in your harness to make sure they are clean and well connected also.