I have a friend that was nice enough to change my alternator for me. I wasn't present during the swap and I cant figure out what connects to the other side of this connector ? Its a 2004 VDP and snaps onto of the top of the air filter housing

 

PaulB, odds are, you are looking at the battery maintainer plug that when the car gets shipped around the world, the shippers plug your car into the ship and it gets put on a battery maintainer. Once the car gets delivered, that plug never gets used again. Tuck it back where you found it and call it good. If you have something not working, then we can revisit this plug.

On a side note, you are posting in the 2010+ XJ section, not in the more appropriate 2001-2009 XJ section.