You should be able to see where it is plugged into the block (on the passenger side), and from memory, it has a silver fairly heavy duty heat shield that goes over the cable. I can see mine from the passenger side wheel well, and you should be able to trace the wire forward to find the plug. Make sure you test it before you need it, mine needs to be replaced.
2008 E350 RB 6.0 - Bullet Proofed, MG 4x4 Conversion
2009 E350 RB 5.4 - Camburg Kit (Sold)