Take it from someone who spent their childhood spending part of every summer on 3 day roadtrips (one way) in old cars...in the 80s! Old cars do not even have the OPTION of air-conditioning. They have air VENTS, but that's hardly the same thing (they just stir the hot air around really).
So, please stop writing about how the boys are all sweaty because the air conditioning is "broken"...write instead that they are all sweaty because they are driving around in a car that was built in 1967!
PS: Yes, I do realize that improvements can be made to old cars - such as installing a tape-deck, which they have obviously done. But until I see canon proof that they've taken the time to install an air-conditioning system, I am going to continue to be annoyed by this.
ETA: Both auriliawestlake and dairygirl pointed out in comments that AC was invented in 1939 (MY MIND IS BLOWN) and that it was actually an option when purchasing a 1967 Impala...so, I guess I've been way harsher on fic-writers than is actually warranted. That being said, AC was an expensive add on, and for some reason I doubt Sal Moriarty (bible-giver-outer-extraordinaire) would spring for an expensive add-on. Basically, I'm still going to go by the assumption that the Impala does not have any add-ons unless I'm told differently (like the tape-deck)....I just spent far too much of my childhood driving around in non-AC old cars with working and lower-middle-class people who couldn't really afford the car, let alone expensive optional features.
Also, the Impala would have had lap belts (that are probably never used)...and is an automatic transmission...and has a bench seat....and yeah, I think that's everything covered in comments. Feel free to chime in if I've missed something.
ETA (AGAIN): Ok, auriliawestlake also pointed out that the automatic transmission would have been an extra add-on when purchased from the factory - so there is evidence that the original owner went for the luxury extras. So, I PROMISE TO BE LESS HARD ON PEOPLE...but personally, I still would rather assume everything is "factory-standard" unless otherwise shown or told.