I also can't have the radio on in my car if I plan to actually think. It's hard for me to even imagine how people are able to concentrate with active music playing or, worse, a television show running in the room. I'm always reminded how different I am every time I stay at one of those hotels that has a communal free breakfast area. They invariably have TVs tuned to those 'happy babble' morning talk shows and most people seem to have no problem working, reading, or carrying on a conversation with it running constantly into their brain.
Apparently, some of us are just made differently. I have an occasional multi-hour drive I need to do and I will sometimes listen to music, a good podcast or an audiobook but I'm actively listening to it. Most often though I drive in complete silence and just think. Some of my friends are surprised by this and can't imagine just sitting in silence that long. However, I love it.