[TerranC]

In that case, a few more possibilities:

1) The woman opened the door for the stranger (easy from the inside) - wife was either already dead or was the woman and was killed by the stranger.

Taera already implied this, and I already posted that that implication was wrong.

2) Stranger to who? Most readings will assume that the riddle means stranger to the man, but that isn't necessarily the case. I haven't met this woman, so she is a stranger to me. The woman could thus be the stranger, and have already been in the car. With such a case, the wife would have already been dead, or the wife was the woman and died from heat stroke/heart attack/whatever (if a dead person can qualify as a stranger).

Close! You've got the concept of the answer, but not quite there.

3) If stranger doesn't imply human, it could refer to an insect/spider/rodent/whatever that got in; cars aren't usually air-tight. If the woman was the wife, she could have died from something while in the car (perhaps a bite). If the woman was not the wife, the wife was already dead.

It's assumed that there are no such things in or near the car.

4) The stranger could have gotten in if the woman rolled down a window. Then, either the woman was the wife and the stranger killed her or the wife was already dead.

Nope.