OP, is there anything on the display while AM/FM are selected? Some rides are programmed to ignore static and play silent if a signal strength is below a certain level. If no display, radio is dead.
If you do have display, change the antenna out (swap with another vehicle if possible). Most common cause though is a broken/bad connection on the antenna wire. Depending on signal strength, FM (due to higher frequencies) can sometimes be picked up off the rest of the cable, but AM needs the complete antenna to work. Make sure your antenna wire is connected to both the antenna and the radio, run a continuity test on the wire. If this still doesn't solve the problem, it's likely the electronics inside the radio have STB.