Just recently got a 99 XJ and was just fixing a few things and came across this plug. It wasn't connected to anything and I can't see anything it would be connected from. Comes from back by the firewall and ends by the coil. Everything that needs to be plugged in appears to be plugged in. Wondering if anyone has any clue. Also everything runs like it should. No issues.

 

its in the area of where the cruise control actuator lives. I'd guess its the plug for that.

 

You may find others that aren't connected. Typically, they are for options your XJ doesn't have, or may be missing. Veeb0rg mentioned CC. Fog lights, outside air temp, under hood light are some additional examples in the engine bay. Not to say you don't have them.

 

I don't have CC so that could very well be it. I did not realize that they still had plugs even if you din't have certain things. I appreciate the input!

 

It may be for the battery temp sensor that later XJ's have in the bottom of the battery tray. I guess the quantity of wires in the plug would help narrow it down. If it is 4 wires than it is likely the cruise control.