I would generally agree with Val, and add that there are additional variables to consider as well. Seeds in general start to accumulate genetic damage at about 98*F, but clearly don't just die immediately since we have weather hotter than this and still have viable seeds producing through these spells.
How were the seeds packed? If they were insulated in something like boxes inside boxes of cardboard or any newspaper involved, they'll be better off than if they were sitting in the sun. Were they buried in other things? Were they in a separate trunk? You could test the conditions by trying to replicate and putting in a thermometer in morning and checking around 2pm for max temps.
The age of seed and type of plant matters too. Old seed will be farther along the tail end of the germination rate curve already and so will be more likely to die in this situation. Newer seed holds up better. Something that evolved to deal with desert heat might be longer to take damage. Bigger seeds might hold up better for more thermal mass, and take longer to 'cook' for all I know.
How hot does your car get? Interior matters; mine is dark blue and it gets hot enough in there I have used it as a passive solar food dryer @165*F with the windows cracked facing south. 175*F with windows up. If your car has light colors inside, it won't get quite that hot. If it is black leather, it will get hotter. If it was facing east/west or north and you have a smaller rear window, it won't take as much thermal energy as it could have.
How long were they in there? From solar food drying I can tell you it's really from about noon to 3pm that are the hottest and temps can be double digits outside that period pretty quickly where I am. I still had seeds grow from tomatoes I sundried this way. If you didn't end up with popcorn in your seed bags, there is hope. If you are in Phoenix though, RIP roasted seeds.
