I just got my C63 the back seat has three seat belts but I only see two plugs in the back? I tried feeling around for the other but no luck so I thought I would ask. Thanks

 

I'd say the receptacle is jammed in between the seat cushions.

 

There should be two right next to each other in the center closest to the passenger side (assuming US spec); one is tucked into the seating and the other shouls be sticking out more.

Unless you're literally missing the buckle...then I'm not sure what to tell you...

 

My C63 sedan was new from a dealer and the female side of the middle seat beat was missing.

I was going to take it back to the dealer but before I did that, I took the bench seat out and found that the female end hadnʻt been pulled up when the bench seat was installed.