I recently bought a Pioneer 1590DVD and later came to know that these HUs cant play MP4. It can only play .avi that too with a max resolution of 720x480, anything over that will give you an error message "video resolution not supported".
I think if you read the manual it says mp4 can be played but exactly what kind of special mp4 is not mentioned, I tried quite a few times but failed, even mpg files are supported but not playing, due to unknown reasons, I yesterday created dvd from mpg files from windows 7. And dvd plays fine
