I am unable to receive most AM stations, fair reception on FM and Sirius is fine. Ford Service Department blamed it on a bad speaker?
Bad speaker??...doubtfull, verify that with a CD or ipod playing.
It is possible that water has gotten between the antenna and base...just unscrew it, clean off any corrosion, spritz with a little white lithium grease and replace. (grease is optional)
If still no improvement, check that 12V to ground is present at the antenna (blue wire seen at base of antenna in the trunk)
Beyond that...hound Ford to fix. Has it always been that way, or just started??