I'm in default on a judgement involving a car accident. My son was driving my car, and he was given a ticket for failure to yield.

Whan you say you're in default of a judgment, I assume you're referring to a judgment entered against you in a civil suit brought against you by the other driver; the other party's injuries would have nothing to do with your son's ticket. If I understand you correctly, a civil judgment has already been entered against you on default, which means that you probably missed your chance to raise these issues.  I'm not familiar with Georgia law in particular, but in general you can only have a default judgment vacated under two circumstances.   First, if you were not served properly.  If so, the Court had no jurisdiction over you, and any judgment must be vacated.  Second, if you were served properly, you can only get the judgment vacated if you can demonstrate both a good excuse for defaulting AND a meritorious defense to the claims.  If you were served and just ignored the lawsuit, you're probably out of luck, even if you have a meritorious defense.  But if, for example, you didn't respond to the complaint because you had a heart attack and were in in the hospital, the Court may vacate the default and allow you to defend the case.