lani used to scream in the car, whether it be a 5min trip or a 5hr trip, we finally worked out that it was all to do with the sun or the glare shining in her eyes, we got some of those window sox and bang she was fine, even now at 19mths old she crys out as soon as the sun hits her eyes, so now she wears her sunglasses in the car like her mummy lol.

sorry it probably isn't helpful info but i just thought it was interesting, as it took us quite a while to work it out, my poor ds used to cover his ears the whole time we used to drive cause of the screaming.