K.,
I just had to comment because my son is the exact same way! Up until he was about 6 or 7 months old he was wonderful in the car. Something seemed to change overnight because ever since then he gets so upset when we are in the car for more than 15 minutes. I gave up trying to "figure out" what was wrong - his straps were not too tight, his seat is nice and cushy, he is forward facing and can see me or my husband, etc...and just kind of decided that the only way to get through it was to get through it.
I used to shy away from going places with him because of the production it would turn into - and I just hate to hear him cry like that. But, really, the only way to get him used to the car rides is to just do it. I make it a point to go somewhere with him nearly every day so that he learns that the car is not a bad place. Its getting better - his threshold is much longer now and I think he is starting to get that when we go in the car we end up somewhere fun/different. I am just going to keep at it and see what happens!
Good luck! Take care!
T.