Does the manual specifically say that if the headlight blows out, the Zener diode is suspect? I looked over the wiring diagram for your bike, and it appears that the only function of the Zener is to provide a stable voltage to the turn signal circuit - it needs this, or the flashing rate would not be stable or reliable.
From what I see, the headlamp is on a seperate circuit (black wire), but of course, the diagram is not 100% clear on this.
If you were measuring the voltage after the light was already blown out, you were reading what is called the open-circuit voltage. While 16V is way to high for a 6V lamp, the actual voltage under load (with a good bulb) might be much lower. Maybe the bulb just burned out on it's own - they do that. I also see somehting odd in the diagram - it looks like the taillamp is in series with the headlamp.. kind of odd. If the taillamp is missing/burned out, then the headlamp would not light. Again, the diagram is not clear on this.
The diode itself should look a lot like a short bolt, but it has an electrical terminal (with a wire) sticking out of it. It might be attached to the frame itself, in a location where it's hard for water to get at it - and difficult to see. On a Motobecane, it's attached to a heatsink, which is mounted below the headlamp.
Hope some of this is helpful.