Sounds like I had a similar experience and I agree that the car thing is not something you want to start as a habit! My daughter never napped during the day for about the first 4 months of her life but slept great at night almost from the day we brought her home from the hospital. She also cried A LOT during those months of her life! Funny enough that echoed how she was in my belly. She was pretty still all night and ALL day she'd be busy moving around even 'til the very end when she was almost 2 weeks overdue!
There are 2 things that worked for me. One was that I used a baby sling (kind of like a Moby Wrap in the cradle style) and she would fall asleep instantaneously, probably because she was so exhausted anyway. I really do think she wanted to be close to me and hear the familiar sounds of my heartbeat and belly gurglings. On the other hand, at night she had absolutely no problem sleeping by herself in her basinet, in her own room, all through the night waking once or maybe twice to be nursed.
The other thing that worked is that finally around 3 1/2 months, she accepted the pacifier. I had tried earlier and she would just spit it out and get angry. The day she finally took it, she was in her playpen, I just quietly slipped it in her mouth and within 1 minute she was sucking and falling asleep. I definitely was sending up lots of thanks to the powers that be! I'm not a big fan of CIO for a child so young. I did it a few times when I was at my wits end, but never longer than about 20 minutes, I just couldn't take it.
These days she sleeps like a champ during the day (she's 16 months) at least 2 hours twice a day or 3 - 4 hours in one go. I don't even have to rock, sing, hug, etc. I just say it's nap time, give her a pacifier (she only gets it in bed), lie her down, give her a kiss and walk out of the room. Hope this helps!