AM Radio does not work?

Let's check the antenna first and if that is okay the radio is no good, I have seen the antenna go bad and cause this issue. Here is how to test and change out the antenna. Check out the images (below). Let us know what happens and please upload pictures or videos of the problem so we can see what's going on.

Here's what happened:

i bought a used head unit from eBay and tested it and no difference.
i also bought a used antenna and there was also no difference.
however, i was able to obtain Kia Sorento antenna and its able to pick up many AM stations although sound quality is not good.
Any ideas? maybe Ebay sold defect? or cable problem? something else?

I would test the alternator to make sure it is not throwing AC voltage. This guide can help, also check the ground to chassis and the ground to the engine:

https://www.2carpros.com/articles/how-to-check-a-car-alternator

Switch the voltmeter to AC and it should show "0" also raise the engine RPM slightly while testing. Please go over this guide and get back to us.

Alternator appears to be fine. Besides, radio always has a problem whether the engine is running or not.
i suspect cable is the problem. Could you give me a diagram for radio cables?
Do you have any other suspects you can think of?

It does sound like grounding issue, here is the radio wiring and a guide to help you test the connections.

https://www.2carpros.com/articles/how-to-check-wiring

Also, how did the antenna testing go? Check out the images (below). Let us know how it goes.

antenna appears to be fine.
By the way, could you also give me the location of the radio and antenna cables?

The diagrams in the images above are the only images they give us for the antenna. If you have any questions about the service, please upload images so I can see what's going on.

can you tell me where you get all these images? is there a book available?

Yes, they are from AllData or Mitchell 1 both have online subscriptions.