That last sentence was supposed to say
EMI rather than
EFI. Fat fingers on a lap top.
EMI is Electrical Magnetic Interference. EMI comes from magnetic fields which come from conductors carrying electrical current such as your spark plug wires or the overhead transmission lines in your neighborhood. This really drives AM signals nuts. Does not have so much of an effect on FM signals due to the frequency they are transmitted at.
One more thing I forgot to mention in regard to this is that your radio has an "antenna trimmer" adjustment which I believe is located under the RH tuning shaft. This was for fine tuning of mast length to give optimum reception. It is a little tiny screw that you turned while tuned to a station. You adjusted it for the best sound (signal strength)
Another interesting thing about AM/FM radios in 1st gen Camaros (or any car for that matter) is that the majority of cars that came equipped with these were sold in large metropolitan areas. Here in the North East FM broadcast was rare. I remember only one station that broadcast FM in the late 60s which explains the rarity of factory installed AM/FM radios. However, if you look at places like Southern California, FM was fairly popular in the late 60s so it was not unusual for cars to be factory equipped. Cars sold in smaller demographics rarely had them. I remember my father buying a brand new 1969 Impala and it had an AM radio with a separate eight track player that hung under the RH dash. I thought that was the most incredible thing I had ever seen. I used to sit in the car in the garage playing the eight track! Those were the days