safing sensor location

[hamburglar]

My car is throwing a airbag code of 21
I replaced the rear seat and just some route cleaning and the car is throwing this code
Where is this safing sensor located?
I know nothing of air bag systems

 

[hamburglar]

thinking about it I used fuse #7 for a blown fuse. Looking up some diagrams online for the fuse panel some say that it is not used. But, I did find one that says air bag module.
I am going to replace it tomorrow regardless.......but I don't see how that code can come from a missing fuse...
like I said i'm clueless