So this info might be a little dated…
Back when I was installing AM/FM radios…FM was just becoming popular. 
Inside the 8 track opening, towards the back, there was a little screw adjustment for the AM antenna. Adjusting the screw until the AM station came in the best.
I’ve been out of touch with AM radios for a very long time…I would guess there would be some info in the owners manual IF technology hasn’t replaced that adjustment screw. Worth checking into.