I was unable to see the image that Audiojones posted but I assume it was something similar to what's shown here:
https://swling.com/blog/2017/06/mystery-solved-remember-the-8-track-fm-radio-converter/
The interesting thing is the FM radio converter cartridge has its own internal antenna, so what purpose would the antenna on the back of the 8-track player serve and how could it operate as an antenna for a cartridge inserted into the player unless there was some electrical connection between the two? Perhaps the AM radio cartridge was designed in a way that allowed a connection to the antenna to be made when the cartridge was inserted.