I STILL build rear chassis harnesses and all the small sub-harnesses for most FJ25/40/45 trucks. I just stopped building main cowl harnesses due to time, lack of some special connectors on the 1974-83 trucks, and unfortunately money. The amount of time it takes to build and test a full harness set is measured in 10's of hours. If I charged standard shop rates of $75.00 an hour, a 1974 or later FJ40 harness set would be over $2400.00. Most folks just can't afford that...
The reality of most FJ40 main cowl harnesses is that unless they have been in a fire, or literally cut into pieces, they are pretty easy to repair. Simply untaping a harness will reveal the wires under the tape to be in great condition. It's usually only the engine compartment wires that are exposed to heat that need replacing.
On the issue above. That WestBar three wire to two wire conveter uses simple diodes to do the conversion. Not only would that cause the "bleed" issue you have, it would also have resulted in pretty dim trailer lights. The diodes drop .7 volts just doing their job. The trailer lights would have had at best 11 volts on them and probably more like 9 or 10 volts due to voltage drops at connectors and old wire having a higher resistance. If you want to tow with your FJ40, use a converter like the Curt 59196 POWERED converter. It does require a power feed from the battery, but isolates the wiring on the trailer from the FJ40 wiring. So a short in the trailer wiring will not take out the FJ40 wiring. This converter powers the trialer lights directly from the battery so the trailer lights are much brighter compared to the diode based converters.