The deck light is driving me crazy. Went up the mast again to do some additional tests. When the bulb is attached, I wasn't seeing any voltage measured across the bulb or when I tested the positive terminal and grounded the negative to the mast.
Once I removed the bulb from the circuit, I found house battery voltage (13.something with the charger running) when measuring between the positive wire and a ground point on the mast. When testing continuity between the ground wire and the mast, I found an open circuit. I verified that the bulb terminals showed continuity, so it should be OK. It seemed like the ground wire was the culprit and the issue wasn't up on the mast after all.
I was finally able to track down where the ground wire terminated. I had first thought it was grounded through the mast, but after a bunch of detective work, found 3 possible wires that could be the ground in the cabinet under the mast where the other connections are. And the connection looked rather corroded.
Hoping the corrosion was the issue, I cleaned up all the connectors and terminals on the terminal block. I put everything back together using dielectric grease to help combat corrosion. I tried the light again...and nothing.
Not wanting to climb the mast again...and not knowing what I would do if I got up there...I tried thinking up possible scenarios that I could test from the connections in the cabinet. I was able to determine one of the green ground wires was the one for working steaming light and assume the other two, which were crimped together in a connector, were for the anchor and deck lights.
Since both the anchor and deck lights are out, I decided to remove the crimped on connector so I could perform some additional tests. There is, after all, a chance that with both lights out and the common point being that connector that it could be the issue. After removing the connector I could individually test each circuit. I checked for continuity between the positive wire for the deck light and the two unidentified green wires. Both showed open circuits which seems to confirm the bad wire theory. I also tried with the anchor light and it showed an open circuit regardless of which ground wire I tried (this was not unexpected since I hadn't gone all the way up to check on that light at the top of the mast yet...but I was looking for a miracle bit of evidence at this point).
So, it seems that power is getting up to the bulb, but isn't making it back down to ground. The connections up on the mast look reasonable, so it seems that it may be a break in the ground wire somewhere in the mast. I wouldn't guess that this wire would fail though...as it is routed inside a sheath and well secured at all points I could inspect.
I may try replacing the connector up on the mast as a last Hail Mary before I bite the bullet and try pulling new wires through for the light. Why is nothing on this boat ever easy to fix? Maybe if I sleep on it I'll come up with some obvious problem I have missed.