Re: GT 150 Tach install
I just bought a tach for my 1990 GT100, and there was an actual plug and play harness for it that plugged into the connector below where the key goes on the throttle unit. I think you can get it here on iboats, or on fleabay. Iboats is better though.
EDIT: Come to think of it, if you search on here there is a thread with a question I started same as yours. Someone directed me to the same answer and I think there was an actual part # there too...
EDIT #2: I was bored so I looked up the old thread for you. The suggestion was
BRP# 0174732. I'm not sure if that will fit your rig, but it would be a good place to start looking. I have as yet to install anything since I'm buried in snow up to my arse up here, but I did verify that it will work on a 1990 setup on my boat. g/l.