This old thread has been very educational, and helped be configure the heater in my newly acquired 2011 Nissan Leaf. I followed the "de luxe" option 3 instructions and with less than $10 in components, and about 1.5 hr for soldering, I now can control the heater with the AC button.
Here are a couple of notes that may be useful for anyone trying to do this:
1. Make sure that the MOSFET you choose actually triggers at the voltage produced by the voltage divider. The original schematics called for a 1N7000 MOSFET, which I could not find. I used a 2N7000 MOSFET instead, sourced from Amazon. This particular device, requires at least 3V at the gate to operate as a switch. The schematic created by BetterLeaf, calls for a 220kOhm resistor to ground at the voltage divider, dropping the voltage by x10 to about 1.2V at the gate and therefore fails to switch the 2N7000. In my second attempt, I paid closer attention to the parameters, and I used a 1MOhm resistor for this part. Now the divider produces 1/3*12V=4V at the gate, and nicely triggers the MOSFET, thereby showing 330Ohm to ground, to pin #37.
2. The space on the front of the PCB is really very small for all the components. There is very little room for extra long leads, so please plan accordingly. Before your final soldering, test to see if the PCB will still drop-fit in its case, and if the buttons can be pressed freely.
3. For safety, please make sure that you use the large 1MOhm resistors at the RC circuit + voltage divider side. Applying 12V directly to a MOSFET gate without any resistor, will result in smoke and risk very expensive damage within 1-2 minutes.
Otherwise it is a great and very simple modification, that brings some sense back to the ventilation system for the early Leaf (ZE0).
Thank you to everyone who has contributed to this thread!