It's really YMMV. Most who offer it pride themselves on being flexible enough to do everything, and couldn't give a shit less about your car as long as your dick's clean. If she can do it in a room, she can do it in the car, assuming it's big enough. Missionary in the back set of a smart car with a 5' 10" girl isn't likely to work.
One word of caution - if you are going for bbbj/cim, make sure you keep a towel and/or some wipes handy. Unless you have someone who will stay on to the end, there will be at least some mess to clean up on your leg and on the seat. And if they don't swallow (spitters are quitters!) find out what they plan to do with it first. If they are going to open the door and spit, make sure the door isn't hard to open. Also turn off the dome light in advance at night. Or bring napkins for them.
Oh, and if you have an SO who might be in the car later, triple check they don't leave anything behind, and allow lots of time to get the smell out first.
If you want stories, feel free to PM me.
