The trim ring just snaps on, so some gentle prying will be necessary to remove it.
Maybe try bowing it from the top and bottom to determine where the tabs are.
Once one tab is loose the whole trim ring will come right off.
Do other sources work?
You should have AM reception even with the antenna unplugged. The whip antenna is for FM only. The AM antenna is built in.
I wonder if the tuner portion of your stereo isn't working at all.
2013 Classic 30 Limited
2007 Silver Toyota Tundra Crew Max Limited 5.7 iForce
2006 Vivid Black Harley-Davidson Road King Classic
1999 Black Nissan Pathfinder LE
WBCCI #1811, Region 6, Unit 56