I wonder if you can specifically talk to someone at perhaps a budget rent-a-car, or a credit union car buying service, or even a good dealership - if you can find one! I was thinking that they buy used cars all of the time, there is no reason they can't detail a used car for you with the products that you specify, not using the standard fare.

Also, I would think that allowing the car to air out for a week or so before you enter it would help immensely.

And of course, I would specify that you want your own, carefully chosen mechanic to inspect the car, put it on the rack, and give a reasonably clean bill of health - if you are buying used.

We are snowbirds and have traveled from MI to the FL Keys and had to deal with this since I first was told that I had MCS in 1993. I never want a new car just before we head south. We get a new car every 3-4 years. Here are somethings that have helped me but certainly not the complete answer you are looking for.

I have found that luxury cars don't seem to have as much off gasing as less expensive cars. Leather seats seem to be better for me. Not good news for those who cannot afford it.

I have been lucky with the car dealers that I have worked with. I go in a month ahead of time, pick out a car and ask if they can put the car in the show room with the windows down. With the way the market is they might even do it until you purchases a car. They did tell me that people might me sitting in it, I didn't care. Most cars sit in the lots and are never opened unless they are test drivin or sold. I even had one dealer that had an old empty building that they put it in for me.

Buy at the end of the season, nothing worse than a car right off the line. Ask the sales person to open the windows before you arrive the day that you have to drive it home and have someone else drive it home if you can until you can get some of the smells out. We are lucky to have a pickup also and I will use that for a month or so.

Keep the windows down during the day out of the garage as much as you can.

Wipe down the inside of the car often with what ever you can use. I use Shaklee Basic G but I always wear a mask when using it. When you see the fog on your windows that is off gassing.
I put a room size air purifier in the car and run that for a few weeks an hour a day.
Avoid driving the car on sunny days, if you can. The sun seems to bring the gas out.
Wear a mask while drivng a car that is not "clean" with the windows open as much as you can. I use my mask from I can Breathe.
Would have loved to have sold our last car (it wasn't leased) to someone that needed a "clean" car.
I know that some of these suggestions just won't work for everyone, it is what works for me. It also depends on how healthy I am on how these suggestions work.
Good Luck

We bought a new car with the lowest toxic chemical score in its category listed on www.healthycar.org . It is now fine for me, but not very fine when we first brought it home. (This is the stuff of nightmares for someone who is chemically sensitive like me.) The odors (perfume, after shave, laundry products?) of potential customers and/or sales people who test drove it before we bought it remained on the surfaces and would not air out. Finally, we washed down all of the interior surfaces with (tolerated) liquid soap and water twice and after that I had no problem. Also, even though the car is great now, I still wear a dust/mist mask (E.L. Foust) for potential mold from the air conditioner - a small price to pay for a tolerable car.

Keep trying for used directly from original owners, where they have never had them "detailed" nor carpet shampooed. A little old lady who kept it in her garage. Three years old should be enough to be outgassed for some cars. Much older and it has more chance of having been detailed. Also, you could alert used car dealers to call you when they FIRST get a car in, BEFORE they do ANY cleaning or detailing, and not do any and not shampoo the carpet or spray. Nada! Not even vacuum.