I was actually talking to someone at work about this today, Creepy. Anyway, he says that the plug you are talking about is indeed for the LEDs in the SEL's and SES's. He has to double check to make sure but this is the guy that rebuilds and sells focus', same guy I got my headliner and black interior from. He just picked up an SEL to work on and is going to check it out and let me know, and once I know I'll let you know. Provided you don't find out before hand, and if you do, tell me.
Get your trunk or hatch LED conversion kits here http://www.focusfanatics.com/forum/showthread.php?p=4542652#post4542652
"2nd Gen problems"
I've spent too long looking into the abyss. It's starting to look back.