Replaced both my yellow stock headlights with a pair of Spyder lights and noticed this plug and harness and have no idea what it's for...educate me!

 

Maybe for fog lights.

 

My truck is a 2001 and didn't come with fog lights. Wonder if the harness was in all trucks as an option. Not sure.

 

My truck with fogs has that harness. Can't find it in the diagrams I think it's just there lol. There is one behind each headlight

 

Long story short, while routing a salvaged harness, I determined that's a ground joint (cluster) .

Ground Joint example

Long story LONG, have an '01 also without factory fog lights. My dad gave me the fog light harness from a Nissan and the actual lights off another Ram. I kept reading that there is wiring for fog lights on our trucks even if there are no lights installed. Try as I might, never located it so I used what came off the Nissan. I went to a local junkyard and got the headlight/fog light switch off either a Durango or Dakota. Ran only 1 new wire from that thru the firewall to connect to a relay on the salvaged fog light harness. Best I recall, I put a jumper wire inside the relay so fogs stay on with high beams; we live in a rural area where deer abound & I don't want a hoof or rack in my face.