"Port" or "Input" for AM is done through the antenna.
If only the AM is not working and this is the second unit with the same problem, I would assume the antenna is not properly ground or not grounded at all.
How is the reception for FM stations on this radio compared to those that you know are good in your truck, car or home?
If you can put the radio on an AM station that you know, put it on that station and have some go outside and grab the antenna. You should get some different noise or better reception.
Let us know what you find out. You may even want to go to a local car parts store and buy a real cheap antenna and plug it in the radio yourself and see what happens. If you do this, hold the base or the antenna.
HAPPY CAMPING!! :D
Daryl and Sandy
2013 Eagle 314BDS
2012 Ford F150 Platinum, 4 x 4 Crew Cab with Ecoboost