If she's startling herself awake, she might enjoy being swaddled. You have to be careful in a carseat, obviously, but it can be done. Here's an idea from a mom on a message board here on mamasource:
http://www.mamasource.com/article/ideas-for-swaddling-in-...
"I swaddle my little boy in his car seat with a blanket. I just take the blanket I use to swaddle him normally, and lay it in his car seat. I set him in on top of the blanket, then buckle him, and then I swaddle him by wrapping the edges of the blanket around him and tucking them under. That way he's buckled properly, and also comfortably swaddled at the same time. It keeps him calm."
They also mentioned a specially-made car seat swaddler here:
http://www.kiddopotamus.com/p_swdltrvl.php
Definitely something I wish I'd had for my little one. Babies that age just like to feel snug and car seats are SO big for them at first. I used to roll up a couple of plush receiving blankets and wedge them along my babies hips and shoulders (after she was buckled, of course) to give that extra bit of snugness as well. This is actually a great thing for safety as well, since there will be less room to bounce around in the event of a crash.
We also used these pillow things for head support and I CANNOT recommend them strongly enough:
http://www.toysrus.com/product/index.jsp?productId=2403258
Nothing keeps a baby awake more than having their head flop around at every red light and turn and these pillows are a great fix. The only place I've seen them at is BabiesR'Us so hopefully you have one nearby. They velcro around the car seat straps and stay in place nicely.
I completely understand the temptation to nurse while driving (and I'm sure you've already gotten some grief over your confession here) but I just wanted to encourage you to consider pulling over instead. Nursing will only set you back a few minutes and the baby will be SO much safer. Also, the sooner you get her interested in a good pacifier, the sooner she'll be able to soothe herself.
Good luck to you, Sweetie. And hang in there!!!