It's pretty unlikely any car, much less an old Cruiser, is air tight enough to asphyxiate you. Think about it, if cars were that airtight you'd get people passing out, especially with a full passenger compartment.
But you will want to open windows if you can (e.g. it's not raining) because it will get really humid inside and the windows will fog. Think back to junior year in high school in the back seat of dad's Impala...
'91 Toyota Pickup
'09 Kawasaki KLR650
'12 Gunnar Rockhound 29
"They say the test of literary power is whether a man can write an inscription. I say, 'Can he name a kitten?'" -- Samuel Butler