Has anyone ever gotten comfortable enough with their client or lady to venture outside of normal incalls?  

You could hold a veritable orgy in that thing...

I do not have that 67 Chevelle anymore but I do have an SUV with a pretty large backseat.....lol.

I can't imagine that you would be hard-pressed to find a willing participant. I also think that you'll discover that car sex isn't as great as you are imagining (unless it's the thrill of public sex that you're after). If you have a wagon/SUV/truck, anything where you can flatten the seats, then you're all good. But a compact/subcompact? Forget it! The bruises you end up with won't be due to the sex but from bumping your head/elbows/knees trying to get into position.

Lord, how I remember...some of those words from song---Against The Wind)
Those warm spring nights at the drive in show with those Teenage Dreams---Katy) Putting hands all over , under, the taste of lipstick---Hey!  wasn't there lead in those things!!! Shoe did put "lead" in my pencil;P~~~ I don't think either one of us cared)

Lets look at this in an analytical way

It just happened between dinner and making it back to the hotel. Holding hands walking to the car, quick kiss at the car, kissing pressed up against the car, back door opened, kissing as we got into the back seat, skirt up and pants down it was urgent and felt like I was a teenager! Caught our breath, started giggling, rearranged ourselves and went to the hotel. Laughing and holding hands to her room where we snuggled in. It felt like it was our secret and extremely hot! Thanks for raising that memory for me.

of course it would be a little tight in the backseat of my car, but where there is a will, there is a way

Did it once in a car with a woman I knew from the office where we both worked. We had run into each other at a club in Nashville.  This was in 1976.

Find a nice quiet place to park the car and put a blanket or sleeping bag on the hood under the stars.

Some how we got connected and felt comfortable with each other. In your case you will have to find a hobbyist confident enough to venture out and not nervous ninny.

For under the stars, a summer night deep in the countryside on a bed of moss is wonderful.  Needs to be warm enough out so that you feel the warmth from the earth and moss.  I don't think this weekend would be a great time for it.

My guy wanted to take us out to a movie after a lil' rendezvous at my place, love to have the opportunity to rock my black thigh highs, he loves boots, sexy pants and top to show off my curves and cleavage, we had an awesome night on the town, loved how he couldn't keep his hands off me all night, even got all hot and steamy in the car,  and when we got back to my place, we went back for seconds . . felt like high school again

Never done it with a provider, but man that reminds me of a girl I dated in High School.  I worked summers at walmart, she would pick me up on my lunch break and we would drive to a "secluded" spot less than a mile from the store and get it on in her corolla.  Start with making out and then we would slide the front passenger seat all the way back, her in the seat me on my knees, slide her umbro shorts to the side and bang away for my lunch hour.  Got busted once by a cop but he just told us to move along or he would tell our parents.  

have a couple nice memories of that...one was in the parking lot of a nice little mexican restaurant near Chapel Hill right off 15/501 near the Lowes.  After a long drive back home, me and my GF (attractive Indian from Mumbai) were hungry and decided to stop in for a bite.  Feeling more randy than hungry, we ended up in the back seat, her legs and feet in the air, my butt against the window, and other positions.  Problem was that it was middle of winter and the inside got 'steamy' and windows fogged up like hell.  Ended up with a nice butt print on the back window.  Saw some family just staring in disbelief, rushing to get the kids in the door.  Needless to say, we got more than a few stares when going inside and a couple of 'attaboy-nods' from the guys who were wishing....