I can not sit in a car for any length of time before I NEED to get out and move around. Also I have pillows for every chair I sit in, this includes my car, my DHs car, my DR chair, my couch. I have a pillow under me and behind me. I have several pillows in my bed to support my back, under and btw my legs. I walk every day and I stretch several times per hr. I haven't been on any long trips in decades due to my inability to sit still for long. I must look like I have ants in my pants. I am never comfortable sitting in any kind of a chair. I fell off a balance beam in HS, I landed on the crossbar. That added a whole new dimension to my life!!
FibroNana mentioned how her private area goes numb, I get that quite often. It is unnerving, to say the least.
If you have a long ride, the best I can offer is pillows behind and under, take many breaks and walk around. I hope it is a pain free ride for you.
God bless. Alice.