Sorry I can't be of much help for you, we're having a similar problem we have been for about a month now, but it's only at night.

We've tried toys to look at, no toys, different toys, leaving the interior light on, having someone sit in the back with him, music on music off... we've had no luck sorry.

I'm going to stalk this thread to pinch any ideas OPs may give you

I know how you feel though the crying is awful, DS gets so worked up he coughs and splutters and takes a while to settle down even after we're out of the car

I have 2 teenagers and at least twice a week I have to go out after dark to drop off and pick up from soccer trainings, a good 15-20mins each way, not a long trip ordinarily, but but feels like hours with a distressed baby!

One other suggestion I was given was to check out the capsule/car seat my friend found that her toddler had hidden the remote control for the TV behind the cover on their capsule, once they got it out bub was fine, turns out she was very uncomfortable seriously though it happened!

Good luck