Hi folks, just an update for the curious. Thanks to all your helpful suggestions, my Becker is rewired, installed and working great.
In addition to your help I called Ed Ebel at Becker Radios in N.J. He confirmed that although the rear bracket acts as the ground to the firewall, he recommends attaching a wire to the center screw and grounding to the chassis. Also confirmed the two plastic connectors with set screws are for the trigger wire for the power antenna.
The last obstacle to overcome was adapting connectors for the speaker output. It was a feeling of relief when I attached the battery and turned the knob - the antenna raised, the dial lit and music filled the interior. I finished assembling the dash and took it for a spin. Couldn't have finished the job without your input.
Thanks,
-jakl