Friends,
Putting my 2005 R53 back together after doing some pretty major maintenance on it:
-1) Alternator
-2) Power steering Pump
-3) Right hand CV Axle, Hub, and Carrier
-4) Harmonic Balancer
-5) 100% rebuild of my TSW MDM Brake Calipers, Porterfield R4 Pads, and new Wilwood rotors
-6) Ripped out Rear seats

And probably some other things I am forgetting....

But one thing I cannot remember what its for is this plug just right behind the radiator?

What is it?? (I found the other end and connected the two at least)

 

Connects to this:

 

Knock sensor.