we had this problem--and it started around 9 or 10 weeks--with our first child. Here on Mamsource, it seems to be a more common problem with first kids, so I'm a little surprised to see it for a second child. Does the other child sit where the baby can see them?
Probably just a case in point that kids are all unique ;). Sorry.
I never solved it, really. It was way worse at night, so I stopped going anywhere at night. I had read a little about early infant trauma and so I actually for a brief period stopped going anywhere at all--because if you enforce the trauma it programs into their brain, whereas if they get a trauma but then you remove that stimulus, their brains are growing so fast and they only retain *patterns*, that they drop that trigger, entirely. I couldn't stay home THAT much (I did erase a water immersion trauma from another one of the babies by not bathing her for 4 months ... babies don't really get THAT dirty and she is a little waterbug now). But after a few weeks the car-cry was mostly only if she was tired or hungry, and I could make shortish daytime trips, or feed her up so full she would be falling asleep before we got in the car and usually then she would fall asleep while we drove for longer trips. Although sometimes this would only give her the calories for a REALLLLLY long cry, because she hated to fall asleep unless Mommy was holding her.
I really didn't go much of anywhere when she was between the ages of 2 months and about a year, though ... so, not to want to freak you out (because there is NOTHING like the stress of listening to a baby in SUCH distress, being techincally close enough to be able to hold them, and being unable to ACTUALLY pick them up--and then you are supposed to be a competent DRIVER during all that!?!?!) ... but just to say, you might only be able to mitigate the problem.
Another thing is to try different kinds of music. My first, liked classical (which I had liked alright but never so much that I would listen to it all the time). When I discovered *she* liked it (would quiet down), we listened to it for two years straight ... until the next one came along and liked *jazz*. I don't even remember how I figured that out, because i did NOT like jazz--like, *at all*--but it was a sleeping pill for him--30 seconds and he was out. I learned to like jazz ;). (Really for real I did--once I was listening enough to appreciate it.) Never figured out with number 3--but she wasn't the cryer in the car (she was pretty socially oriented since birth and I suspect the other two kids were enough for her).