The car gets quite hot during summer under the sun.

That is so true Dytecture. I use the car for starting seedlings. Plastic is placed over the small pots soil so water evaporation is at a minimum. After germination we transplant them. I read somewhere that water + heat makes germination faster. So i thought maybe increasing the heat will decrease the time for the plant to actually start to grow. So far, that idea is working

What type of greenhouse/sun room are you adding? Have you made a decision yet?