Speaker question

[dmjlambert]

No diodes, use relays that you can cleanly and completely connect/disconnect from the player or radio. Speaker signals are AC current, not DC such that you can direct with diodes.
Regarding power, my thought is to cut power to the AM radio when the relays give the speaker to the 8-track player, because you are not supposed to power up the AM radio without speaker attached.
It would probably take several relays because when connected to the AM radio the speakers should be connected in series with each other to add up the impedances (4 + 4 =8 ohms) that the AM radio needs.