Not sure why my initially installed inverter went out but I always wondered why it was advertised to have a yellow override wire and it was not on the unit when it was received. The isolator allows my second battery to charge when the car is on and disconnects from the primary battery when the car is off.
If you are putting in a second battery, I suggest using this isolator. A link is below.
Check out my initial article ADDING A SECOND BATTERY TO MY PRIUS – PART: 1 where I installed the isolator.
The benefit of the yellow wire is so if your primary battery dies, you can trigger the isolator to send power from your secondary battery to the primary in case you need to jump-start your vehicle.