The woman was the stranger - she was already in the car, hiding (the trunk, perhaps?) (Alternately, the woman was the stranger and had the keys). There isn't anything in the riddle itself that says the woman was the man's wife; the wife could already have been dead at the outset.
The woman is not a stranger, as referring somebody as a stranger means that he or she wasn't previously known to the protagonist.
Plus, the woman could have been the man's girlfriend, or Roommate, or Close Friend, and etc.
Another possibility, requiring a few typos on your part: car should be plural; they were driving seperately. The woman's car broke down, and the man went for help for her. He locked up his own car, and found the stranger in her car, not his. Again, the wife was already dead, as there is nothing to say that the woman was the man's wife.
[Edited] Edit: There was a typo on the riddle, but not like the ones you suggested.
Another possibility: the woman was the man's wife, but opened the door for the stranger (she had the keys), who killed her.
Although possible, that's not the answer.
Another possibility: There are methods for getting into a locked car without damaging it physically. The stranger could have gotten in with one of those methods. The man's wife was either the woman (who the stranger killed) or was already dead.
Thinking too much into it, aren't you?
Could be possible, but not the answer.
This one isn't hard, it's open-ended; there are many possibilities, but of course only one is what happened. There is no real way to choose which one is correct without already knowing the answer. If you care to contradict me on this, find a problems with all the answers I gave that are wrong.
Indeed. As you said, there's only one answer.
[ July 31, 2003, 23:36: Message edited by: TerranC ]