Does the head unit have a wire to connect to a lights circuit so it dims/brightens when the headlights are turned on/off? A few times i've put HU in newer cars I always cause this sort of problem from connecting it to the wrong place!
Try disconnecting that wire and see if it makes a difference, or keeps blowing the fuse!
If you have tapped into the light for the ash tray it could be incorrect and earthing through there.
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