One of the great surprises is that there are a number of large detailed manuals that we need to be familiar with, and that they often get revised in mid-build. It's a time and brain sink....
Simply put a wire between the seat heater switch and a contact input of the EMS, but per a note from Dynon, put a 10K resister in-line from the switch and EMS. This will drop the input voltage to the contact input, as you only needs 5 volts to change the state of the widget. It will take 12 volts, but lower is better.