If heat is the problem, you could try insulating the plants. Try putting them into or under a large cardboard box. Mist the box in the morning before you drive, getting the cardboard as wet as you can while still not causing it to collapse. Have some visquin plastic sheet underneath, so water does not collect on the floor of the trunk. The evaporation of the water will help cool the inside of the box. To be more elaborate, you could line the box with newspaper, making another layer of paper (or cardboard, if that is feasible) so there is air space between the inside and the outside of the box.
In effect, you are making a cooler to house the plants.
I have no idea if this would work, but it seems like a possible technique.
PK