Something like this should work:
After the get out waypoint, make a get in waypoint close by and set it's condition to "incar" or whatever name you want. Have the get out waypoint run this script:
@player IN vehicle
man assignasdriver vehicle
incar=true
AFAIK you can also use an action to get the guy in and out of the vehicle like this:
man unassignasdriver vehicle
man leave vehicle
@player in vehicle
man assignasdriver vehicle
man action ["get in",vehicle]
@man in vehicle
incar=true
This way you don't have to use get in and out waypoints, just run one script to do it all. Just make sure the next waypoint's condition is still "incar" or else as soon as he gets out of the car he'll start walking to the next waypoint.