cjm's photos show exactly where that support bracket goes. It runs from the steering wheel support bracket and attaches to the extra piece of metal that hangs below the center part of the dash. But I don't see that piece on your photo JP.
Maybe its just the angle of your picture, but it looks like that extra piece is missing? That would be a problem, since that also ties in to the fascia support bracket that braces the dash center to the floor. The center fascia support ties everything together (i.e. supports the dash to the floor, and through that angled rod also stiffens the dash center and the steering wheel bracket).
The radio mounting bolts also go through the holes that cjm's photo shows.