[impala L78]

Bill - You mentioned that after several repairs, your AM/FM radio does not work as well as you feel it should. Perhaps, it may be necessary for you to make a final adjustment yourself, once you have installed the radio. I believe that this procedure is in the original owners manual.
Tune the radio to a distant station (FM). Remove the tuning knob. There is a slot to insert a small flat tip screwdriver. Use the screwdriver adjustment to get the maximum volume that you can on this distant station. This adjustment (if necessary), should improve reception on your local stations.
Also, those vehicles that had a factory AM/FM or AM/FM stereo radio should have come from the factory with a one-piece solid stainless steel radio antenna that was approx. 30 to 31 inches long. If I remember correctly, this length antenna was supposed to be ideal for FM reception. If you have a regular telescoping antenna, try adjusting the overall height to the above dimension.

Regarding the location of the multiplex unit for AM/FM stereo. For 1965 (full size cars), it was mounted under the dash. In 1966, this was also true unless you also ordered the optional 4 gauge panel that was located under the dash. In this case, the multiplex unit was mounted on the console door.

Thanks to all who continue to supply great info. here.

John