1984 mercedes 300SD W126

today out of nowhere my car would not start. i would turn the key and nothing. my battery is good i double checked it. so i thought it might be the gear, because the car would not start in anything other than P or N (right?). so i played with the gear a lil bit with no luck.

now when i turn the key. there is a buzz comming from the box mounted on the driverside fender. i think it is the glow plug relay? when it buzz's does it mean that it is bad? do i need to change the whole thing of just a part of it? what is this box called? i need help thanks for any advices