Just thought I'd pass this on. Maybe it has already occurred to you, maybe not. It didn't occur to me until baby #4 who, like his siblings, hated the car.
When they start to fuss in the car, I put on a radio station that is only static (non working station), and I turn it up LOUD. 9 times out of 10 it calms them almost instantly. Around here, I find the FM station give off better static (white noise) than the AM stations.
Hope this helps someone.