Just saw this. Bought my E34 last November and still trying to find my way around it as I do minor fixes...
Where would I find this trailer plug. If it’s still there, I want to get at it ASAP.
There are probably two of these connectors in the much-taped-up factory installed wiring harness between the engine and the panel in the cockpit. There is one where the engine-related circuit wires are gathered up and were fed into one plug, in the engine compartment. Then there is a plug close to the back of the panel where the same plug was installed.
Then... then.... (pause for dramatic effect).... there was a connecting wire bundle - same color coded wires - and this connector was likely offered in a couple of stock lengths by Universal/Westerbeke as supplied with the engine package. The builder (EY in this case, and there were a number of other high volume builders, like Catalina) picked the bundle link that fit the largest number of their boats with similar engines and panels. The wiring could have been the same for several of the 80's models like the 32, 34, 35, and the Olson.
The chance that the length of the connector was "just right" was pretty statistically low, and I know that there was quite a coil of excess under our cockpit for our M25XP.
Since not only were the plugs grossly undersized, some of the wires were also undersized for the planned current, and any extra length also reduced the current flow potential. (Sigh... mutter... mutter... )
While I fault EY to some extent, I also fault Universal/Westerbeke even more. It's not like Ohm's Law was some sort of new fangled unproven idea in the 80's.
And, welcome again to the site. There is a LOT of good information here about your boat model as well as similar ones with the same parts. For instance our electrical panel is the same model as yours and another larger Ericson.