Anybody know what this plug is? I was changing my battery today and noticed it--it was unplugged. This is on an '03 disco II with SAI.

 

Could be the horn on that side. Scroll through the D2 Electrical Library and reference the size, shape, location, and wire colours to confirm.
There's also a temperature sensor in the radiator in that area on some trucks.

 

It kind of looks like a fog light plug. Do you have an aftermarket bumper without fogs that a PO would have disconnected?

 

@greisinb Looks exactly like the light plugs I found on mine, I have my Aux lights plugged into them. Check my thread here https://landroverforums.com/forum/di...lights-102559/

Also check the right side of the instrument panel switches do you have on with a lights symbol on it

 

The plug is probably still there even if the truck was optioned without fog lamps, is that even at thing on an ‘03? I know it was on pre-facelift models but my assumption was that all North American models had fog lamps standard.

 

2003 and 2004 years do have fog lamps, but maybe there were models that omitted them.