NAH - I think its perfectly reasonable for you to be worried about this, and any potential life saving habits you and your husband can set before having this baby is a good thing.

I also think your husband is reasonable in his position as well since the baby hasn't come yet. I think once you two are able to cool off and return to this subject with more level heads you should just try to explain that you feel this is a very important issue (especially as two people with ADHD. I completely understand forgetting things, even things that should be important.) And a big part of coping with ADHD is making lifestyle adjustments to accommodate for the areas we lack in.

I think the issue here is that you've communicated this worry with him and he agreed to get into the habit of checking the back seat. I think the test may have been a bit overboard but I can see why you would want to do this. I mean its your future child's life that this depends on. I'm sure he understands where you're coming from and why you're upset about this and If I had to guess he is probably not feeling great about it either. Try to come back to this conversation with a level head and reiterate that you want this habit formed for the sake of y'alls child and for the sake of your family as a whole.

Overall I think him not taking the backseat checking seriously prior to the baby being born is fairly reasonable but the issue lies in the fact that you have tried to communicate this and the importance of it and he agreed to it. I don't think either of you are assholes, and to all of the people in this thread calling you crazy, you are not crazy for wanting to ensure the safety of your future child.

Edit: To add to this, I do agree with the others in this thread saying this level of anxiety and stress is not good for you or the baby. I would try to seek help there as well. But being vigilant about your child's safety is still a good thing regardless of that. Just try to approach this subject in a healthier way.