The vent on my SD (1981) is right in front of the drivers side rear tire. It looks like the bell of a trumpet sticking down near the bottom of the body (from underneath). Its sorta hard to find but you will come across it eventually. Mine had a bunch of dirt covering it up.

If the location is confusing to you, to the drivers side rear tire. Then, get down on the ground and look up, from underneath, at the bottom of the body directly in front of the tire well area. Shizam...there you go