Are you looking for an internal wiring diagram as how the AM radio works (or does not)?
A few long years ago, I went to a local electronics swap-meet, and found a vendor there selling off his collection of diagrams and trouble shooting guides for the big five (yes, there used to be five) auto maker radio versions. I got them for $5 and promptly gave them to my friend who went and equally promptly lost them all. "You gave em to me?" Out there in the garage someplace I think.
These items are pretty cheap, as no one likes to work on the radios anymore. Zero challange. Your AM still may not work, but it has to be a challange to them. If you take the unit to some Radio/TV shop, be prepared for Just the bill and little on the explaination into the process. My friend used to insult the connection wires and replace them with or w/o a connector. But he was cheap.
Wm.