I’m sure it crosses over somewhere but don’t know which fixture it would be spliced in.  Your friend here is a fox and hound aka tone generator and probe. $20 from Amazon  clip the tone generator on the dead hot lead and use the probe to follow the wire under floors, behind bulkheads..... till it stops (hopefully at a fixture). 

Les

Here is what this device does.
I have no affiliation with this product or company and I am not endorsing this particular product. There are many others that you can choose from.

Many years ago I had same issue with port side lights too. Turns out I had sprung a leak in my bilge hose at the aft end of the boat just before it exists the hull on the stb end. This is under the stb cockpit aft locker where you might store fenders, etc. The water found its way into the corrugated conduit carrying the wires for the port lighting and sat there eventually corroding the wires until it broke the connection. It took me a while to figure that one out. Good luck.    

Basically it crosses from starboard to port in the bow. Look at the  schematic in the manual. If all the lights work on the starboard side up to the bow, but nothing from there on I would suspect that the last light that works is where the problem lies. I had a Cat 30 (1984) and the bow lights were also part of the circuit and it came apart in the anchor well
Additional comment note if your bow light works then one would assume by schematic that the black wire is OK to where it leads off to the bow. One could suspect that if the junction is bad at this point then the lights after that will  not work. If indeed the connection is good then the issue is between that point and the next light in the string. based on schematic.