Could there be a problem with the power feed to the instrument itself? It sounds like connecting the masthead module is drawing too much power and whatever supply filter caps are discharging. With the module already connected, switching power doesn't let it recharge, only applying power to the instrument without the masthead.
I would measure voltage at the instrument with and without the masthead connected, such as on the Seatalk
"pass-through" connector. If there are other Seatalk
instruments, try isolating them and powering this one alone.
Another possibility is that it is not ever actually reading windspeed, just responding to a transient glitch when the transducer is first connected, then settling down to zero.
Also, what part of the cable "shows 2 ohms"? Each conductor, end-to-end, or round-trip pairs, or what? That sounds fine for a bare conductor, providing it's not shorted to another, or ground, or anything else, and that the same goes for each wire in the cable. If that's measured through the transducer, it may not mean much, depending on the position and speed of the cups and your meter's response.
You might also try connecting the transducer directly to the instrument, bypassing all the ship's wiring