do you have tint on ur back window. It's possible that it could be affecting your aerial power. The tint sometimes contains led which can interfere.

Also, from what i remember about Alpine deck's in the past (4-5 years ago) they had pretty shitty FM tuners in them, they are awesome when it comes to CD quality or iPoof connectivity, but Radio reception has been poor. I have only had 1 Alpine deck before and this was the case, but I have had friends who have had them, and have also not been happy with radio reception.