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 Pickup - Imelda
'08 Tacoma TRD - Donna
'09 Kawasaki KLR650
'12 Gunnar Rockhound 29
"Our task must be to free ourselves from this prison by widening our circle of compassion to embrace all living creatures and the whole of nature in its beauty. Nobody is able to achieve this completely, but the striving for such achievement is in itself a part of the liberation and a foundation for inner security. " --Albert Einstein