Jim,
Circuit board #1 in the rear run box, the PCB that Eric is referring to, is a known failure point for the 2006 and newer Patriots and Marquis. When PCB1 is failing a “Eng Comm Failure" notification will be initiated, and the engine will shut down.
The remote start and front/rear selector switches are mounted on this board.
Search for this, “2008 Navigator; Intellitec 73-00825 rear run box board bad. Workaround?” and you’ll find some troubleshooting and workaround suggestions. Also, suggestions for repair of PCB1.
Circuit board # 6, rear run box. When the ISO LED is illuminated it is indicating that the Big Boy relay is energized. Even when no charging is taking place, the Big Boy will remain energized until the surface charge of both battery banks has dissipated, both battery banks must drop below 12.8-volts. This can take several minutes depending on the 12-volt load.
Look at “Coach Assist->Common Problems->Battery Charging Systems” for a good explanation of the operation of PCB#6.
-Scott