AVIC-Z150BH I bought this unit and installed it late last fall so didn't get a chance to test it out with the top down. I'm really disapointed in the brightness of the display. In map mode with the top down I can't hardly see the display at all. I have the brightness and contrast turned all the way up and makes no difference. I have a Garmin gps on my motorcycle and a Garmin gps for the car that is way brighter than this unit is. Even in the music mode screen I can barely see it unless I put the top up. Had I known this would be so dim I wouldn't have bought it. Oh well, it is what it is. Still sounds great anyway. 

Do you have Daytime Running Lights (DRL) on your vehicle? Your HU might always be in night mode.

Also, try changing the display/screen angle to reflect less light. I think the Z150 has this feature - the Z110-Z140 does.