As I've said before, looks like an accessary cord for a block heater. I'm heading to my shop shortly, I'll see if I can find one and try to post a pic. Can you trace the origins of the first purchaser or check the BMW site to find out what optional equipment it came from the factory in the USA with. They may have had to sub a NA harness into the car if there was a shortage. I've gone through my manual schematics and can't find a three pronged connector anywhere in the car. But, won't stop till you do. JIM