Good afternoon, Jim. We faced a similar problem 2 or 3 years ago. Our Raymarine wind instrumentation began providing erratic directional readings even as it showed a reasonable windspeed. Eventually, it stopped showing any data. Given that sequence, it was pretty easy to tell that it was the sending unit (the instrument at the top of the mast) that was failing, but I still followed a standard troubleshooting process.
The first step was to examine/trace the cables exiting the mast to identify which was for the wind vane. For the Raymarine, the windvane connects to a multiplexer that also receives the data from the depth transducer. The multiplexer then sends data to the SeaTalk (Raymarine's proprietary take on NMEA 2000) network that feeds our various displays. So, I made sure that the individual wires from the wind vane's cable were terminated to the multiplexer in a sound manner, with no corrosion and secure, well-made crimps. It's probably not relevant to your setup, but I think there is an LED or two on the multiplexer's circuitry that gave feedback indicating the connections were good.
Regardless, this let me know that we should have good signal from at least the base of the mast. (Like you, I wanted to determine as much as possible without going up the mast, and to avoid it altogether if I could.) The multiplexer in my setup sends both wind data and depth data via one cable to the SeaTalk/NMEA backbone for distribution to the other instruments. The depth data was functioning correctly, so that indicated that the cable from multiplexer to backbone was sound. But that's the key, trace from the base of the mast to the backbone and ensure all connections along the way are solid.
Although not specifically relevant to your case, I was then able to discuss my troubleshooting up to that point (which included resets and attempts to recalibrate that did nothing to correct the problem) with the person who installed my electronics. Based on what I described, he advised that it sounded as if the instrument had failed at the masthead. Because the instrument was only about a year old, he made contact with Raymarine and had them provide a new one under warranty. He indicated that even if they hadn't already made the decision to replace it under warranty, they'd have likely sent the replacement, which could be swapped, and then the old unit could be returned for evaluation. I guess even the manufacturer is sensitive to the fact that a lot of us are sailors, not mountain-climbers, and would prefer to minimize the time up the mast. (Also, there had apparently been some manufacturing issues with a series of the Raymarine wind vanes that resulted in a number of failures, so I guess it was no surprise that another one had died. They didn't even want the old one for a post-mortem.)
I was lucky enough to have the installer climb the mast and swap the unit for me, and that solved the issue for me. If the installer hadn't been willing to do it as part of warranting his work, the marina I'm at is primarily a repair yard and so I could have gotten one of them to go up. I'm sure there are riggers in your area whom you could hire. If I were in your shoes I would bite the bullet and buy a new masthead unit. A windvane is by nature fully exposed to the elements and somewhat delicate even as it's robust. It has to be sensitive to light winds, but also withstand 60+ kts as well, right? In the sun, rain, ice, & snow. A seven-year-old (or older) unit is probably due to be replaced. I doubt it would be reparable. And even so, you'd then get to pay someone to go up the mast a second time. So, I'd recommend only one mast-climb, swap it out, and if by some miracle the old one can be repaired, keep it as a spare for when the new one eventually dies.
In fact, after acquiring the new masthead unit, plug it in while you're still on the ground and blow on it. Hell, you could make it a party game.
