No AM radio signal on two aftermarket HUs?

[atebit]

I've tried two different model Alpine HUs in my 07 sedan, and have noticed in both cases that AM radio reception is pretty much non-existent. FM radio comes in strong/fine, so I don't think it's a case of a bad windshield antenna connection to the radio.

I originally had the "integrated" AM/FM/CD stereo in the car. I'm almost starting to wonder if the factory radio had a built-in AM loop antenna or something like that, instead of it using the windshield antenna? The only other thing I can think of is maybe there's a separate feed for the AM antenna on the wiring harness that I wasn't aware of.

Not having AM isn't a huge deal...99.995% of the time I'm listening to either XM or the iPod, but in my area there are some decent AM news stations that I listen to every once and a while. Has anyone else had an issue with AM (or even tried to listen to AM) after replacing their stock HU?