72 Imperial backup light

Hi,

The backup light of my 72 Imperial isn't working anymore when I put the car in reverse.
I started looking for the fusebox - relay (I don't know exactly how it is called) but I cannot find it. I thought it was located near to the steering column.

Can anyone help me out here?
Is there a wiring diagram available were I can check al electrics for the backup light?

Thanks!
Patrick