I will have to differ with Jameshoward. The original Becker radios that were installed in the SLs were attached to the firewall with a rear securing clamp (on a flat metal piece of angled steel), which was attached by to the bolt at the middle of the back of the radio with a nut and washer. This champ is spring loaded and attached itself to a plate/ledge welded to the firewall. This held the back of the radio in place while the front was held in place by the face plate (not the chrome surround of the radio). The clamp was at the correct position/height for the firewall ledge, so when the radio is placed into the dashboard opening with some force, the clamp would open enough to slide onto the firewall ledge, thus securing the radio in back of the dashboard. This also made for easy radio removal. I do not believe that these clamps/steel flat angle pieces are still available from Becker. My suggestion is to look for old Becker radio that still has the securing clamp attached to it. Hope this helps you.
Now as for your amp, that attaches to firewall behind the glove box.
Robert Hyatt
Williamsburg, VA.
1970 280SL, Red on Silver,4-speed, Euro spec; 1999 C230 Kompresser Sport, Black on Black; 1990 Miata, Black on Red, 5 Speed