It should be appropriate in a relationship for you to hear his stuff and agree that in fact it does make sense to override the rear A/C, or hear it and say, no it doesn't so he'll need to return the car.
I agree. I feel like I should be able to say, "Look H, this is the car that I drive. I'm responsible for toting S where he needs to be and I don't want it to be stinking hot in the back for him. Our current van is not dying, it's not urgent that we replace it right now. I don't want to have to deal with having to sell our current van and pay sales tax and all the other stuff involved for an interim van that doesn't have the one thing I want. Since it's my car, I feel like I should be able to have my one requirement met w/o argument from you."...
... without H responding, "Yeah, I know, but....."
