Why am I making sure I don't have a powered antenna? Sorry, I didn't catch that from your post.
Sometimes when car companies use a short antenna, they will use active electronics to boost the antenna's reception.
Most radios have a legacy wire that will energize when you use the radio. This is normally used in older cars with antennas that retract and extend when the radio is on. So you turn on the radio and the antenna raises up out of the fender (or wherever). New cars with stubby antenna with active electronics use the power coming out of the radio which normally would be used to extend the antenna to power on the active electronics. These active electronics compensate for not having a huge whip antenna. And some cars, if that's not powered, you will get no radio reception.
It's a common problem if you have active electronics for your antenna.
It's a moot point now that you have FM working. Glad you got it hooked up. But in case someone else reads this.