The indicator was for the EU diesel vehicles with the optional diesel fired engine heater for preheating the diesel engine in cold weather conditions.

In North America, we usually use what is called an electric block heater.

A 120VAC block heater, but not the power cord, was often installed in the NA spec vehicles that had the cold weather package and the 4.4L V8 engine.

By about 2008, that engine heater indicator location was now a green headlights on indicator for NA spec vehicles.

The link is to a file re the 120VAC block heater.
https://www.disco3.co.uk/gallery/dis...um=3801&pos=39