Had my wheel liner out and noticed this unused plug located behind the washer fluid bottle. What is it for?

 

You will find lots of them thought the truck. They are for different market configurations/options. This one would probably be for tire pressure monitoring antenna.

 

That looks to me like the plug for a headlamp washer pump if you had one. Like this pic I found on the internet:

 

I pulled my LF liner out and saw that too. I don't have fogs and assumed the lads at Solihull said "WTF, an SE without fog lights? I'm just going to leave that harness hanging here." But now that I see your pic, I guess that wouldn't even reach. So yeah, I guess probably what @P-Bod said above.

 

Yup, mine does not have the fogs either. Wish it did though. Center consul cooler but no fog lights. Go figure.